drm/radeon: fix sparse warning: Should it be static?

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: Dmitry Rozhkov <dmitry.rozhkov@linux.intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>