drm/i915: Split atomic wm update to pre and post variants

Try to update the watermarks on the right side of the plane update. This
is just a temporary hack until we get the proper two part update into
place. However in the meantime this might have some chance of at least
working.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Clint Taylor <Clinton.A.Taylor@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>