msm: mdss: wait for pp done when switching from ON to LP1

If a low power mode switch is initiated while a kickoff is
in progress, the panel could be turned off before the
current frame transfer is completed causing a display hang.
This change recognizes this case and waits for the current
frame to finish before initiating the power state switch.

Change-Id: Ieea32acd67c56a7ed8a83ff92c3b49ad76d70086
Signed-off-by: Rajkumar Subbiah <rsubbia@codeaurora.org>