drm/i915: split intel_cursor_plane_update() 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.

The commit part can still fail, but that should be solved in another
upcoming patch.

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>