drm/omap: fix WB height with interlace

When using WB capture from interlaced source, we need to halve the
picture heights correctly.

Unfortunately the current dispc_ovl_setup_common() doesn't deal with
interlace very neatly, so the end result is a bit messy.

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