drm/mxsfb: Make supported modifiers explicit

In contrast to other display controllers on imx like DCSS and ipuv3 lcdif/mxsfb does not support detiling e.g. vivante tiled layouts. Since mesa might assume otherwise make it explicit that only DRM_FORMAT_MOD_LINEAR is supported.

Signed-off-by: Guido Günther agx@sigxcpu.org Reviewed-by: Lucas Stach l.stach@pengutronix.de Signed-off-by: Stefan Agner stefan@agner.ch Link: https://patchwork.freedesktop.org/patch/msgid/26877532e272c12a74c33188e2a72abafc9a2e1c.1584973664.git.agx@sigxcpu.org (cherry picked from commit f4b29bf7)

This is a straight cherry pick but having this out sooner makes e.g. byzanthum/bullseye more fun with the versions that come in via the Debian archive.