drm/msm/dsi-staging: Add DSI ULPS support during suspend

Some panels requirement to go to ULPS state during suspend.
Add support for this feature. Keep DSI CTRL power module enabled
during suspend for the DSI PHY to maintain ULPS state.

Change-Id: Ib99fa01d75d1c8cd161357897ed25a430d2bba2a
Signed-off-by: Lei Chen <chenlei@codeaurora.org>