rust: drm/kms: Add RawPlaneState::crtc()

Add a binding for checking drm_plane_state.crtc. Note that we don't have a
way of knowing what DriverCrtc implementation would be used here (and want
to make this function also available on OpaquePlaneState types), so we
return an OpaqueCrtc.

Signed-off-by: Lyude Paul <lyude@redhat.com>