OMAP: DSS2: DSI: add option to leave DSI lanes powered on

The DSI pins are powered by VDDS_DSI. If VDDS_DSI is off, the DSI pins
are floating even if they are pinmuxed to, say, safe mode and there's a
pull down/up.

This patch gives the panel drivers an option to leave the VDDS_DSI power
enabled while the DSS itself is turned off. This can be used to keep the
DSI lanes in a valid state while DSS is off, if the DSI pins are muxed
for pull down (not done in this patch).

There will be a slight power consumption increase (~100 uA?) when the
VDDS_DSI is left on, but because this option is used when the panel is
left on, the regulator consumption is negligible compared to panel power
consumption.

When the panel is fully turned off the VDDS_DSI is also turned off.

As an added bonus this will give us faster start up time when starting
up the DSS and the regulator is already enabled.

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