OMAPDSS: HDMI: Replace hdmi_reg struct with u16

Remove usage of hdmi_reg struct to use u16 instead in the HDMI IP header
file. hdmi_reg struct is not really needed, and the same change was also
made for dispc earlier.

Signed-off-by: Mythri P K <mythripk@ti.com>
[tomi.valkeinen@ti.com: updated the description]
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>