media: uvcvideo: Add D3DFMT_L8 support

Microsoft HoloLense UVC sensor uses D3DFMT instead of FOURCC when
exposing formats. This adds support for D3DFMT_L8 as exposed from
the Acer Windows Mixed Reality Headset.

Signed-off-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>