drm/msm: remove the right vlbank event from the event list

This changes fixes an issue with vblank event list corruption.
we should only remove the event corresponding to a crtc from list
for which the thread was queued.

Change-Id: I9b9cef9073ba4cc377a85e21a7257d36e9b92388
Signed-off-by: Rahul Sharma <sharah@codeaurora.org>