drm: Clean up kerneldoc for struct drm_driver

Just cleans up what's there, still plenty missing.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161114115825.22050-6-daniel.vetter@ffwll.ch