msm: mdss: delay iommu mapping of buffers until commit

Currently iommu map call is done in client's context during overlay
play, this requires iommu attachment to correctly map buffers.
This mapping can be delayed until the actual commit and avoid
unnecessary delays in client's context. Also, only unmap buffers if they
have been mapped, instead of checking if iommu is attached.

Change-Id: I6b26acb3ed843f03b42df1386ee7bdd412365aff
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>