drm/radeon: fix possible lack of synchronization btw ttm and other ring

We need to sync with the GFX ring as ttm might have schedule bo move
on it and new command scheduled for other ring need to wait for bo
data to be in place.

Signed-off-by: Jerome Glisse <jglisse@redhat.com>
Reviewed by: Christian König <christian.koenig@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>