drm/amdgpu: re-work VM syncing

Rework how VM operations synchronize to submissions. Provide an
amdgpu_sync container to the backends instead of an reservation
object and fill in the amdgpu_sync object in the higher layers
of the code.

No intended functional change, just prepares for upcomming changes.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Friedrich Vock <friedrich.vock@gmx.de>
Acked-by: Felix Kuehling <felix.kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>