drm/radeon: add error handling to radeon_vm_unbind_locked

Waiting for a fence can fail for different reasons,
the most common is a deadlock.

Signed-off-by: Christian König <deathsimple@vodafone.de>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Jerome Glisse <jglisse@redhat.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>