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