drm/amdgpu: reserve the pd while cleaning up PRTs

We want to have lockdep annotation here, so make sure that we reserve
the PD while removing PRTs even if it isn't strictly necessary since the
VM object is about to be destroyed anyway.

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>