msm: mdss: wait for kickoff before executing mode switch changes

Currently, as soon as the mode switch for video-cmd mode request
kicks in, few software structures are changed immediately. This
might cause issues when the previous kickoff is in progress with
the kernel thread. Add wait for kickoff before executing mode
switch to avoid such issues.

Change-Id: I2646d00d64416523da9c936715e9af9a2b258eeb
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>