drm/vboxvideo/modesetting: Provide function names for prototype headers

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

 drivers/gpu/drm/vboxvideo/modesetting.c:11: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 drivers/gpu/drm/vboxvideo/modesetting.c:54: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
 drivers/gpu/drm/vboxvideo/modesetting.c:87: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Cc: Hans de Goede <hdegoede@redhat.com>
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/20210602143300.2330146-27-lee.jones@linaro.org