OMAPDSS: Add interlace parameter to omap_video_timings

Add a parameter called interlace which tells whether the timings are in
interlaced or progressive mode. This aligns the omap_video_timings struct with
the Xorg modeline configuration.

It also removes the hack needed to write to divide the manager height by 2 if
the connected interface is VENC.

Signed-off-by: Archit Taneja <archit@ti.com>