Impact: make symbol static.

Fix this sparse warning:
  drivers/gpu/drm/radeon/radeon_dp_mst.c:332:38: warning: symbol 'mst_cbs' was not declared. Should it be static?
Signed-off-by: NDmitry Rozhkov <dmitry.rozhkov@linux.intel.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>