OMAP: DSS2: DSI: ensure VDDS_DSI is disabled on exit

The panel drivers can leave the VDDS_DSI regulator enabled, even when
the panel is disabled, to ensure that the DSI pins are powered.

This patch ensures that VDDS_DSI is disabled on DSI module unload.

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