drm/amdgpu: Add helper to wait for BO fences using a sync object

Creates a temporary sync object to wait for the BO reservation. This
generalizes amdgpu_vm_wait_pd.

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