drm/radeon: make MMU_NOTIFIER optional

In cases where MMU_NOTIFIER is not available, userptr will not be
available.  Similar to i915, although not making an exception for
CAP_SYS_ADMIN.

The proposed userspace patches for userptr seem to handle the fall-
back properly, so a userptr-less kernel should not be a problem.

Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>