drm/amdgpu: update the PASID mapping only on demand

Updating the PASID is rather heavyweight and shouldn't be done all the
time.

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