drm/i915: Add an optional selection from i915 of CONFIG_MMU_NOTIFIER

userptr requires mmu-notifier for full unprivileged support. Most
systems have mmu-notifier support already enabled as a requirement for
virtualisation support, but we should make the option for i915 to take
advantage of mmu-notifiers explicit (and enable by default so that
regular userspace can take advantage of passing client memory to the
GPU.)

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452503961-14837-3-git-send-email-chris@chris-wilson.co.uk