udlfb: Make dlfb_ops constant

Static structure dlfb_ops, of type fb_ops, is not used except to be
copied into another variable. Hence make dlfb_ops constant to protect it
from unintended modification.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Cc: Bernie Thompson <bernie@plugable.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190819075236.1051-1-nishkadg.linux@gmail.com