msm: mdss: keep clocks on during kickoff operation

There is deadlock scenario where work thread to turn clocks off is
trying to acquire DSI's mdp_busy lock while holding clk_mtx to turn
mdp clocks on.  Meanwhile kickoff thread holds mdp_busy lock and
trying to acquire clk_mtx to turn on clock.
This patch ensure that clocks are kept on and work thread is not
scheduled while kickoff is in progress so that deadlock will not happen.

Change-Id: Iceb5db431a7f9f0e1f56a0e3c1ced2d08d762b86
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>