drm/i915: Move vblank evasion to commit (v4)

Move the vblank evasion up from the low-level, hw-specific
update_plane() handlers to the general plane commit operation.
Everything inside commit should now be non-sleeping, so this brings us
closer to how vblank evasion will behave once we move over to atomic.

v2:
 - Restore lost intel_crtc->active check on vblank evasion

v3:
 - Replace assert_pipe_enabled() in intel_disable_primary_hw_plane()
   with an intel_crtc->active test; it turns out assert_pipe_enabled()
   grabs some mutexes and can sleep, which we can't do with interrupts
   disabled.

v4:
 - Equivalent to v2; v3 change is now squashed into an earlier patch
   of the series.  (Ander).

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <conselvan2@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>