drm/amdgpu: Fix wrong mask in get_atc_vmid_pasid_mapping_pasid

Signed-off-by: Felix Kuehling <Felix.Kuehling@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Oded Gabbay <oded.gabbay@gmail.com>