OMAPDSS: Fix de_level in videomode_to_omap_video_timings()

In videomode_to_omap_video_timings, de_level is always set to HIGH
regardless of what the vm->flags says. Fix this to set it to high or low
according to vm->flags.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
[tomi.valkeinen@ti.com: rewrote the desc]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>