OMAP4: DSS2: HDMI: Use specific HDMI timings structure

As hdmi has few additional parameters such as vsync and hsync
polarity which is missing in DSS timing structure, define HDMI timings
structure for hdmi to use instead of OMAP DSS timing structure.

Signed-off-by: Mythri P K <mythripk@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>