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

Due to the upcoming atomic modesetting feature we need to separate
some update functions into a check step that can fail and a commit
step that should, ideally, never fail.

This commit splits intel_update_plane() and its commit part can still
fail due to the fb pinning procedure.

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>