OMAP: DSS2: Use vdds_sdi regulator supply in SDI

This patch enables the use of vdds_sdi regulator in SDI subsystem.
We can disable the vdds_sdi voltage when not in use to save
power.

Signed-off-by: Roger Quadros <roger.quadros@nokia.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@nokia.com>