drm/amdgpu: always register an MMU notifier for userptr

drm/amdgpu: always register an MMU notifier for userptr

commit e574c2cd8a6de5804b14b0d7c5fb0e24e39bd4a1 upstream.

Since switching to HMM we always need that because we no longer grab
references to the pages.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Felix Kuehling <Felix.Kuehling@amd.com>
CC: stable@vger.kernel.org
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>