video: udlfb: Constify read only data

Both dlfb_fix and fb_device_attrs are never written to, so
it is safe to make them const.

Signed-off-by: Ladislav Michl <ladis@linux-mips.org>
Cc: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>