omapfb: constify omap_video_timings structures

These omap_video_timings structures are only copied into other
structures, so they can be const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>