msm: mdss: avoid kickoff wait during validation phase

During ATOMIC_COMMIT validation phase, waiting for kickoff
introduces delay where the commit N+1 from userspace waits
on driver's commit N thereby causing fps drop.
Remove this wait during validate phase.

Change-Id: I20ec0f522ba2531df6abdbb04e95cdea19bc2687
Signed-off-by: Nirmal Abraham <nabrah@codeaurora.org>