drm - Fix lock order reversal

drm - Fix lock order reversal

* Lock order reversal caused by holding dev_pager_mtx() across
  the object->un_pager.devp.ops->cdev_pg_dtor() call.  devpgr -> drmslk.

* Move the lock from before to after the call.  Holding the mutex shouldn't
  be necessary across the call.  This also fixes the reversal as devpgr is
  no longer held across the call.

* Fixes a lock order reversal against drm_ioctl() which obtains drmslk first
  and recurses into a device pager operation which gets devpgr.

* Fix a few other incidental bugs that would normally not be triggered by
  the DRM code due to outer locks held by the DRM code.  Plus some formatting
  fixes.