drm/i915: Call plane update functions directly from intel_atomic_commit.

Now that there's only a single path for all atomic updates we can call
intel_(pre/post)_plane_update from intel_atomic_commit directly. This
makes the intention more clear.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>