drm/omap: fix WBDELAYCOUNT with interlace

Vertical blanking needs to be halved on interlace modes. WBDELAYCOUNT
was calculated without such halving, resulting in WBUNCOMPLETE errors.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Benoit Parrot <bparrot@ti.com>