drm/msm/sde: remove extra read of vbif halt ack register

Remove unnecessary extra wait of the vbif halt ack register
since the wait logic already polls this value, it is not
necessary to read it again. Also, if the wait logic does fail,
deassert the VBIF halt request bit to leave the HW in a good
state.

Change-Id: Ia4e2baf3be8d2dc104e8b52157795e32455f1a1f
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>