drm/i915/gen9: Compute DDB allocation at atomic check time (v4)

Calculate the DDB blocks needed to satisfy the current atomic
transaction at atomic check time.  This is a prerequisite to calculating
SKL watermarks during the 'check' phase and rejecting any configurations
that we can't find valid watermarks for.

Due to the nature of DDB allocation, it's possible for the addition of a
new CRTC to make the watermark configuration already in use on another,
unchanged CRTC become invalid.  A change in which CRTC's are active
triggers a recompute of the entire DDB, which unfortunately means we
need to disallow any other atomic commits from racing with such an
update.  If the active CRTC's change, we need to grab the lock on all
CRTC's and run all CRTC's through their 'check' handler to recompute and
re-check their per-CRTC DDB allocations.

Note that with this patch we only compute the DDB allocation but we
don't actually use the computed values during watermark programming yet.
For ease of review/testing/bisecting, we still recompute the DDB at
watermark programming time and just WARN() if it doesn't match the
precomputed values.  A future patch will switch over to using the
precomputed values once we're sure they're being properly computed.

Another clarifying note:  DDB allocation itself shouldn't ever fail with
the algorithm we use today (i.e., we have enough DDB blocks on BXT to
support the minimum needs of the worst-case scenario of every pipe/plane
enabled at full size).  However the watermarks calculations based on the
DDB may fail and we'll be moving those to the atomic check as well in
future patches.

v2:
 - Skip DDB calculations in the rare case where our transaction doesn't
   actually touch any CRTC's at all.  Assuming at least one CRTC state
   is present in our transaction, then it means we can't race with any
   transactions that would update dev_priv->active_crtcs (which requires
   _all_ CRTC locks).

v3:
 - Also calculate DDB during initial hw readout, to prevent using
   incorrect bios values. (Maarten)

v4:
 - Use new distrust_bios_wm flag instead of skip_initial_wm (which was
   never actually set).
 - Set intel_state->active_pipe_changes instead of just realloc_pipes

Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Lyude Paul <cpaul@redhat.com>
Cc: Radhakrishna Sripada <radhakrishna.sripada@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1463061971-19638-10-git-send-email-matthew.d.roper@intel.com