drm/omap: Change the types of struct omap_video_timings members

omap_video_timings struct have the same members as struct videomode, but
their types are different. As first step change the types of the
omap_video_timings struct members to match their counterpart in
struct videomode to catch any type cast related issues.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>