msm: dsi: Turn off supplies in low power state

This change allows certain phy supplies to be turned off
when display transtions to low power state. This change
adds funtionality to check each supplies setting and
accordingly turns off the the regulator if it is
allowed in low power state.

CRs-Fixed: 2009863
Change-Id: I764a8e65246b5de733f27e4f73d966b19039895d
Signed-off-by: Abhijit Kulkarni <kabhijit@codeaurora.org>