drm/radeon: try to enable VM flushing once more

Let's try to fix bugs related to this instead of just disabling it.

Signed-off-by: Christian König <christian.koenig@amd.com>
Tested-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>