msm: mdss: fix race condition between commit and power off

while commit thread is in progress, suspend is called and stop thread
sets the commit_pending flag to zero, but commit thread increments the
flag, due to which while resume pan idle func time out.
check for disp thread before incrementing the commit_pending flag.

Change-Id: I92483a2b9c44cc41c6d31e8a7d3b2a5bfe11fbc9
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>