OMAP: DSS2: DSI: Improve dsi_mux_pads parameters

dsi_mux_pads() needs to know about the DSI HW module and the DSI lanes
used. Split the function into two, enable and disable, which take
necessary arguments, and add empty implementations for both.

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