drm/drm_ioctl.c: kerneldoc

As usual pull it into the drm docbook template, too. And again as
usual I've decided to only document stuff exported to drivers, so all
the old leftover markup from the shared drm repo days lost the magic
** signature.

Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>