msm_fb: display: perform bus scaling updates when idle

Move bus scaling updates to same place where MDP clock is changed which
happens when MDP is idle. This allows MDP sufficient time to finish
fetching buffers before commiting unstaging changes.

Change-Id: Id6e04522928b0f815091e17054fff94e0e111ff1
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>