OMAPDSS: hdmi: Make hdmi structure public

Unify the private anonymous struct in hdmi4.c and hdmi5.c, name it as
struct omap_hdmi, and make it public in hdmi.h.

Signed-off-by: Jyri Sarha <jsarha@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>