Updated comment style to kernel-doc format in drm_device.h

In struct drm_device there are 12 struct members without doc:
- registered
- filelist_mutex
- filelist
- irq
- vbl_lock
- event_lock
- hose
- sigdata
- sigdata.context
- sigdata.lock
- agp_buffer_map
- agp_buffer_token

They all need proper documentation, a task left for someone
that knows their usage.

drm_device is not plugged into Documentation/gpu/drm-internals.rst
as this would create a new load of warnings.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Maxime Ripard <maxime.ripard@bootlin.com>
Cc: Sean Paul <sean@poorly.run>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20190108192939.15255-2-sam@ravnborg.org