OMAP: DSS2: enable VDDS_DSI when using DPI

OMAP: DSS2: enable VDDS_DSI when using DPI

It looks like on OMAP3 some DSS pins need VDDS_DSI to function properly.

This has not been confirmed from TI, but looking at figure 15-1 "Display
subsystem highlight" from the TRM, some data pins come near the DSI and SDI
blocks. This is not very hard evidence, but the fact remains that with the
power on, pixels are ok, and with the power off, pixels are not ok.

It may also be that VDDS_SDI is needed to power some pins, but as normally
both VDDS_SDI and VDDS_DSI come from the same power source, this hasn't
been shown.

It seems that a single driver can only get a regulator once. This patch
solves it by getting all the required regulators in one place, and from
which the submodules then get the regulators they need.

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