msm_fb: display: fix fake iommu buffer drop

During pipe free, some content of pipe is backed up and then after
pipe is zeroed out, the content get restored, in between there should
be no actions to change the content since any change will be wiped out
by the restore.

Change-Id: Ia6da0162d1f9b614682e1fc36edab1db003b42ab
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>