drm/msm: execute pending vblank disable upon last close

Vblank disable is dispatched by delay timer upon file
release.  If all vblank disable are not dispatched by
last close, device may have unbalanced vblank requests
upon next device open.  As a result, before closing down
all mode objects in last close, trigger vblank delay timer
and wait until all requests are executed and cleared from
workqueue.

Change-Id: I56d39e617fc88d40b22740c0178e339c32305549
Signed-off-by: Alan Kwong <akwong@codeaurora.org>