msm: mdss: fix iommu fault due to incorrect buffer unmap

Any buffer that is being fetched by MDP to display on panel should not
be unmapped until there is a new buffer to replace it. Ensure multiple
buffer updates without a commit to display won't cause displayed buffer
to be unmapped.

CRs-Fixed: 420561
Change-Id: Ibe720976024d7739c36bd4f05f9f70a061ed1eb9
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>