OMAP: DSS2: DSI: implement ULPS enter and exit

Entering ULPS (Ultra Low Power State) happens by sending ULPS entry
sequence to the DSI peripheral and pulling the DSI lines down.

Exiting ULPS happens by sending ULPS exit sequence.

We can send the ULPS entry sequence by using OMAP DSS HW's ULPS support,
but we cannot use the ULPS exit support from DSS HW. DSS HW refuses to
send the ULPS exit sequence if it thinks that the lanes are not in ULPS.
After being in OFF mode the DSS HW has been reset, and so it does not
know that the lanes are actually in ULPS.

Thus we need to use the lane override support and manually send the ULPS
exit sequence. Luckily the sequence is very simple.

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