drm/amdgpu: add vm root BO lock before accessing the vm

Add a vm root BO lock before accessing the userqueue VM.

v1:(Christian)
   - Keep the VM locked until you are done with the mapping.
   - Grab a temporary BO reference, drop the VM lock and acquire the BO.
     When you are done with everything just drop the BO lock and
     then the temporary BO reference.

Signed-off-by: Arunpravin Paneer Selvam <Arunpravin.PaneerSelvam@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>