msm: mdss: ensure pipe is halted before freeing buffers

When pipe is being detached from mixer, there is potential corner case
if underrun occurs at the same time which can cause pipe to flood the
VBIF. On the other hand unmapping a buffer requires VBIF to be idle to
sync TLB, so need to ensure pipe halt is done before freeing the pipe
buffers.

Change-Id: I9686b9d7d17d8dc2a8334157cfa8fa23ba5b47ee
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>