msm: mdss: fix issues with transition from LP3 to LP2

When transitioning from LP3 power state to LP2 power state, reset
the flag to indicate that the interface is active again and can accept
updates. This ensures that all the interface clocks are correctly turned
back during the subsequent display update.

Change-Id: Ia3e90a5e9da7432529a9dd5ca3c5690b8d6c2af0
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>