msm: mdss: Avoid dfps update till unblank is complete

There can be cases where MDP is resuming at default fps
and in between dfps update comes before unblank is completed.
This might result in mismatch in fps and blanking
settings in MDP and DSI. Hence avoid any fps update until
unblank is finished.

Change-Id: If4fed28b1d706f38c39fba06fa786eac6d9ed8c8
Signed-off-by: Jayant Shekhar <jshekhar@codeaurora.org>