msm: display: fix iommu page faults for devices with lcdc panel.

Fetch was happening from non-iommu buffer because of wrong mixer
configuration for lcdc panels.

Change-Id: Ie313ce71ff269108271437f758c0dc37884a240c
Signed-off-by: Sangani Suryanarayana Raju <csuryan@codeaurora.org>