drm: Allow drivers to register cursor planes with crtc

Universal plane support had placeholders for cursor planes, but didn't
actually do anything with them.  Save the cursor plane reference inside
the crtc and update the cursor plane parameter from void* to drm_plane.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Pallavi G<pallavi.g@intel.com>
Acked-by: Dave Airlie <airlied@linux.ie>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>