drm: mxsfb: Update internal IP version number for i.MX6SX

The LCDIF present in the i.MX6SX has extra features compared to
the i.MX28. It has however lost its IP version register, so no official
version number is known. Bump the version to MXSFB_V6 following the i.MX
version, in preparation for support for the additional features.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Stefan Agner <stefan@agner.ch>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20200727020654.8231-18-laurent.pinchart@ideasonboard.com