drm: mxsfb: Implement .format_mod_supported

This will make sure applications which use the IN_FORMATS blob
to figure out which modifiers they can use will pick up the
linear modifier which is needed by mxsfb. Such applications
will not work otherwise if an incompatible implicit modifier
ends up being selected.

Before commit ae1ed009 ("drm: mxsfb: Stop using DRM simple
display pipeline helper"), the DRM simple display pipeline
helper took care of this.

Signed-off-by: Daniel Abrecht <public@danielabrecht.ch>
Fixes: ae1ed009 ("drm: mxsfb: Stop using DRM simple display pipeline helper")
Reviewed-by: Stefan Agner <stefan@agner.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/2a99ffffc2378209307e0992a6e97e70@nodmarc.danielabrecht.ch