drm/omap: fix HDMI sync polarities

While implementing writeback support, odd behavior of WBDELAYCOUNT was
observed with the combination of WB capture and HDMI. The result of the
debugging was that the HDMI sync polarities are not set correctly.

The current code sets the sync polarities going from HDMI WP to DISPC
according to the video mode used, which seems to work normally fine, but
causes problems with WB as WB expects the syncs to be active-high.

This patch changes the HDMI sync polarities so that the DISPC always
gets active-high syncs from HDMI WP, and the HDMI core gets sync
polarities according to the used video mode.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>