Revert "ARM: OMAP2+: Remove legacy mux code for display.c"

Commit e30b06f4 (ARM: OMAP2+: Remove
legacy mux code for display.c) removed non-DT DSI and HDMI pinmuxing.
However, DSI pinmuxing is still needed, and removing that caused DSI
displays not to work.

This reverts the DSI parts of the commit.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>