drm/i915: Make compilation of userptr code depend on MMU_NOTIFIER.

Now that unsynchronized mappings are removed, the only time userptr
works is when the MMU notifier is enabled. Put all of the userptr
code behind a mmu notifier ifdef.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210323155059.628690-16-maarten.lankhorst@linux.intel.com