ARM: OMAP4: display: convert display to use syscon for dsi muxing

The legacy control module APIs will be gone, thus convert the display
driver to use syscon. This change should eventually be moved to
display driver from the board directory.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>