drm/omap: Replace struct omap_video_timings with videomode

omap_video_timings can be replaced with the generic videomode in omapdrm
and the omap_video_timings can be removed.

This patch will replace the omap_video_timings with videomode.
With the change we no longer need the functions to convert to/from
videomode and drm_display_mode to omap_video_timings, these can be removed
as well.

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