drm/amdgpu: rework job synchronization v2

For unlocked page table updates we need to be able
to sync to fences of a specific VM.

v2: use SYNC_ALWAYS in the UVD code

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>