drm/omap: omap_display_timings: constify videomode structures

The videomode structures are only copied into the vm field of a
panel_drv_data structure, so they can be const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>