drm: make DRM_UNLOCKED ioctls with their own mutex

drm_getclient, drm_getstats and drm_getmap (with a few minor
adjustments) do not need global mutex, so fix that and
make the said ioctls DRM_UNLOCKED. Details:

  drm_getclient: the only thing that should be protected here
  is dev->filelist and that is already protected everywhere with
  dev->struct_mutex.

  drm_getstats: there is no need for any mutex here because the
  loop runs through quasi-static (set at load time only)
  data, and the actual count access is done with atomic_read()

  drm_getmap already uses dev->struct_mutex to protect
  dev->maplist, which also used to protect the same structure
  everywhere else except at three places:
  * drm_getsarea, which doesn't grab *any* mutex before
    touching dev->maplist (so no drm_global_mutex doesn't help
    here either; different issue for a different patch).
    However, drivers seem to call it only at
    initialization time so it probably doesn't matter
  * drm_master_destroy, which is called from drm_master_put,
    which in turn is protected with dev->struct_mutex
    everywhere else in drm module, so we are good here too.
  * drm_getsareactx, which releases the dev->struct_mutex
    too early, but this patch includes the fix for that.

v2: * incorporate comments received from Daniel Vetter
    * include the (long) explanation above to make it clear what
      we are doing (and why), also at Daniel Vetter's request
    * tighten up mutex grab/release locations to only
      encompass real critical sections, rather than some
      random code around them

Signed-off-by: Ilija Hadzic <ihadzic@research.bell-labs.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Dave Airlie <airlied@redhat.com>