drm/fourcc: Add DRM_FORMAT_Y8

drm/fourcc: Add DRM_FORMAT_Y8

Add greyscale Y8 format.

The 8-bit greyscale format has been discussed before, and the earlier
guidance was to use DRM_FORMAT_R8, as a single-channel 8-bit pixel.

However, adding DRM_FORMAT_Y8 makes sense, we can mark it as 'is_yuv' in
the drm_format_info, and this can help the drivers handle e.g.
full/limited range. This will distinguish two single-channel formats:
R8, which is a RGB format with the same value for all components, and
Y8, which is a Y-only YCbCr format, with Cb and Cr being neutral.

Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Vishal Sagar <vishal.sagar@amd.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Simon Ser <contact@emersion.fr>
Link: https://patch.msgid.link/20260423-xilinx-formats-v10-3-c690c2b8ea89@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>