drm/i915: Add two-stage ILK-style watermark programming (v11)

In addition to calculating final watermarks, let's also pre-calculate a
set of intermediate watermark values at atomic check time.  These
intermediate watermarks are a combination of the watermarks for the old
state and the new state; they should satisfy the requirements of both
states which means they can be programmed immediately when we commit the
atomic state (without waiting for a vblank).  Once the vblank does
happen, we can then re-program watermarks to the more optimal final
value.

v2: Significant rebasing/rewriting.

v3:
 - Move 'need_postvbl_update' flag to CRTC state (Daniel)
 - Don't forget to check intermediate watermark values for validity
   (Maarten)
 - Don't due async watermark optimization; just do it at the end of the
   atomic transaction, after waiting for vblanks.  We do want it to be
   async eventually, but adding that now will cause more trouble for
   Maarten's in-progress work.  (Maarten)
 - Don't allocate space in crtc_state for intermediate watermarks on
   platforms that don't need it (gen9+).
 - Move WaCxSRDisabledForSpriteScaling:ivb into intel_begin_crtc_commit
   now that ilk_update_wm is gone.

v4:
 - Add a wm_mutex to cover updates to intel_crtc->active and the
   need_postvbl_update flag.  Since we don't have async yet it isn't
   terribly important yet, but might as well add it now.
 - Change interface to program watermarks.  Platforms will now expose
   .initial_watermarks() and .optimize_watermarks() functions to do
   watermark programming.  These should lock wm_mutex, copy the
   appropriate state values into intel_crtc->active, and then call
   the internal program watermarks function.

v5:
 - Skip intermediate watermark calculation/check during initial hardware
   readout since we don't trust the existing HW values (and don't have
   valid values of our own yet).
 - Don't try to call .optimize_watermarks() on platforms that don't have
   atomic watermarks yet.  (Maarten)

v6:
 - Rebase

v7:
 - Further rebase

v8:
 - A few minor indentation and line length fixes

v9:
 - Yet another rebase since Maarten's patches reworked a bunch of the
   code (wm_pre, wm_post, etc.) that this was previously based on.

v10:
 - Move wm_mutex to dev_priv to protect against racing commits against
   disjoint CRTC sets. (Maarten)
 - Drop unnecessary clearing of cstate->wm.need_postvbl_update (Maarten)

v11:
 - Now that we've moved to atomic watermark updates, make sure we call
   the proper function to program watermarks in
   {ironlake,haswell}_crtc_enable(); the failure to do so on the
   previous patch iteration led to us not actually programming the
   watermarks before turning on the CRTC, which was the cause of the
   underruns that the CI system was seeing.
 - Fix inverted logic for determining when to optimize watermarks.  We
   were needlessly optimizing when the intermediate/optimal values were
   the same (harmless), but not actually optimizing when they differed
   (also harmless, but wasteful from a power/bandwidth perspective).

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1456276813-5689-1-git-send-email-matthew.d.roper@intel.com