drm/mm: Document locking rules

Drivers need to take care. Motivated by a discussion between Mark and
Rob on dri-devel.

Cc: Mark yao <mark.yao@rock-chips.com>
Cc: Rob Clark <robdclark@gmail.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
[danvet: s/alloc|freeing/modifications/ per Chris' suggestion.]
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1482833457-29592-1-git-send-email-daniel.vetter@ffwll.ch