drm/radeon: add error handling to fence_wait_empty_locked

Instead of returning the error handle it directly
and while at it fix the comments about the ring lock.

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>