msm: mdss: wait for vsync before updating fps dynamically

In the current implementation, when there are continuous updates
to fps, dynamic interrupt completion may fail. To avoid the interrupt
failure, wait for the vsync before configuring dynamic refresh pll
registers and trigger.

Change-Id: Ieade09e54073db5633aaf845885888d7b2c45091
Signed-off-by: Jeevan Shriram <jshriram@codeaurora.org>