OMAPDSS: split hdmi muxing function

Split the omap4_hdmi_mux_pads() function into two parts, one handles the
tpd12s015 gpio muxing, the other handles the hdmi pins.

This is clearer, as hdmi and tpd12s015 are separate devices, and it also
allows us to mux those separately with DT.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Ricardo Neri <ricardo.neri@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>