drm: msm: fix list corruption problem

When multiple worker threads compete to update event_list, with current
vblank_ctrl_worker mechanism, there is one risk which can casue list node
is deleted for twice. This is because, due to the protected scope by
the spin_lock, mutex can exit when one thread is transvering the list,
but another queue thread may continue to add new event to the list.
This brings conflict risk. This patch is to correct this.

Change-Id: Ice31462d196c57ce18d7b998c1a1f0b7feeb08fc
Signed-off-by: Xiaowen Wu <wxiaowen@codeaurora.org>
Signed-off-by: Guchun Chen <guchunc@codeaurora.org>