drm/amdgpu: try to find BO VAs only for the BOs in the list

The other ones don't have any VAs assigned anyway or are uninteresting to us.

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