WIP: Avoid running RADEONLeaveVT_KMS() twice
RADEONLeaveVT_KMS() will un-reference its all-black FB in a timer, but if we end up calling RADEONLeaveVT_KMS() twice in a row, we may try to access those after they've been just freed up, and crash.
Return early in RADEONLeaveVT_KMS() if the VT semaphore is unset.
Bugzilla: https://bugzilla.redhat.com/1674474 Signed-off-by: Olivier Fourdan ofourdan@redhat.com