OMAPDSS: DSI: fix lane handling when entering ULPS

The current code always enters ULPS for 3 lanes. This is not right, as
there could be only 2 lanes used, and on OMAP4 we have 5 lanes.

Fix the code to put all used lanes into ULPS.

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