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 <&lt;davej@codemonkey.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>