drm/msm/dsi-staging: add dsi phy isolate enable config

The DSI controller can work isolated from phy without
pll locking. That allows to verify the dsi controller
specific changes by isolating phy. Add debug property
support for isolating the phy from dsi.

Change-Id: Ibbfa23e0c2fc9e0e4db152cd83c65e0516617d04
Signed-off-by: Dhaval Patel <pdhaval@codeaurora.org>