drm: omapdrm: dsi: Handle pin muxing internally

Don't rely on callback functions provided by the platform, but access
the syscon internally to mux the DSI pins.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>