drm/msm/dsi-staging: enable dsi idle to turn off phy

Phy can be turned off during idle period to conserve power.
Add idle mode support to turn off phy and reinitialize PLL
CSRs upon resume from idle.

Change-Id: If2c11a5a82c839f88b39c9d5d6da97888e4457ba
Signed-off-by: Alan Kwong <akwong@codeaurora.org>