msm: mdss: dsi: enter ULPS after blanking the panel during suspend

In the current implementation, when the display is blanked, DSI phy
is turned off prior to turning off the supply to the panel. This results
in an abrupt transition on the DSI lanes from LP11 to LP00 which is in
violation of the MIPI spec, and can potentially cause panels to go to a
bad state. Fix this by requesting ULPS entry when display is blanked.

Change-Id: I5c3e422cfdd9a398939a7277cd78ed7adb16d76f
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>