drm: Export drm_ioctl_permit to kernel-doc

drm_ioctl_permit is exported but missed a kernel-doc style
documentation.

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20161228143216.26821-5-krisman@collabora.co.uk