msm_fb: display: free iommu buffers for WFD

Iommu buffers from overlay destinated for WFD (mixer2) are
not freed after kickoff. This patch will free those iommu
buffers.

CRs-fixed: 394270
Change-Id: I6b58b5d1831e7c59c5107641b786730511803557
Signed-off-by: Kuogee Hsieh <khsieh@codeaurora.org>