drm/i915: Consolidate top-level .update_plane() handlers

Our .update_plane() handlers do the same check/prepare/commit/cleanup
steps regardless of plane type.  Consolidate them all into a single
function that calls check/commit through a vtable.

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