msm_fb: display: Fix blend configuration for MDP4.1

MDP4.1 blend operation registers are not double buffered and
take effect as soon as they are configured. Configuring these
registers at any place other than between dma and vsync produce
flickers as new blend values would get applied to previous
mixer configurations. Configure blend operation registers just
after dma is over, before vsync.

Change-Id: I377c11222f7a0cd77e98e90edec6e2e1dc0c17ca
Signed-off-by: Pradeep Jilagam <pjilagam@codeaurora.org>