drm/plane-helper: transitional atomic plane helpers

Converting a driver to the atomic interface can be a daunting
undertaking. One of the prerequisites is to have full universal planes
support.

To make that transition a bit easier this patch provides plane helpers
which use the new atomic helper callbacks just only for the plane
changes. This way the plane update functionality can be tested without
being forced to convert everything at once.

Of course a real atomic update capable driver will implement the
all plane properties through the atomic interface, so these helpers
are mostly transitional. But they can be used to enable proper
universal plane support, especially once the crtc helpers have also
been adapted.

v2: Use ->atomic_duplicate_state if available.

v3: Don't forget to call ->atomic_destroy_state if available.

v4: Fixup kerneldoc, reported by Paulo.

v5: Extract a common plane_commit helper and fix some bugs in the
plane_state setup of the plane_disable implementation.

v6: Fix issues with the cleanup of the old fb. Since transitional
helpers can be mixed we need to assume that the old fb has been set up
by a legacy path (e.g. set_config or page_flip when the primary plane
is converted to use these functions already). Hence pass an additional
old_fb parameter to plane_commit to do that cleanup work correctly.

v7:
- Fix spurious WARNING (crtc helpers really love to disable stuff
  harder) and fix array index bonghits.
- Correctly handle the lack of plane->state object, necessary for
  transitional use.
- Don't indicate failure if drm_vblank_get doesn't work - that's
  expected when the pipe is in dpms off mode.

v8: Review from Sean:
- s/fail/out/ to make the meaning of a label more clear.
- spelling fix in the commit message.

Cc: Paulo Zanoni <przanoni@gmail.com>
Cc: Sean Paul <seanpaul@chromium.org>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>