drm/i915: split intel_primary_plane_setplane() into check() and commit()

As a preparation for atomic updates we need to split the code to check
everything we are going to commit first. This patch starts the work to
split intel_primary_plane_setplane() into check() and commit() parts.

More work is expected on this to get a better split of the two steps.
Ideally the commit() step should never fail.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>