drm/mga/mga_ioc32: Document headers do not make good kernel-doc candidates

drm/mga/mga_ioc32: Document headers do not make good kernel-doc candidates

Fixes the following W=1 kernel build warning(s):

 drivers/gpu/drm/mga/mga_ioc32.c:2: warning: Cannot understand  * file mga_ioc32.c

Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210115181313.3431493-3-lee.jones@linaro.org