drm/radeon: don't do mst probing if MST isn't enabled.
drm/radeon: don't do mst probing if MST isn't enabled.

This causes an oops as we haven't initialised the mst
layer.

Reported-by: Dave Jones <fd8cf1a10f352f50bb3508e298f0e0eab0c94338@codemonkey.org.uk>
Signed-off-by: Dave Airlie <f2295d84e358395675bc8031be58672073ae065e@redhat.com>
Signed-off-by: Alex Deucher <08dc22c6156113f2deff178e35e3ed9b24d6af9e@amd.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs