msm_fb: display: Flush pipe registers before staging the pipe

Flush pipe registers before staging the pipe for the Mixer with
pull mode panel interfaces. This fixes the IOMMU page fault/underrun
caused by pipe fetching previously unmapped buffers due to missing flush
calls

CRs-Fixed: 356484
Change-Id: I6bc2e49746a7c3773708796bc1709c75a8afaa4b
Signed-off-by: Ravishangar Kalyanam <rkalya@codeaurora.org>