drm/connector: Pass a drm_connector_state to ->atomic_commit()

Other atomic hooks are passed state objects, let's change this one to
be consistent.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180703075022.15138-3-boris.brezillon@bootlin.com