msm: display: Fix blend configuration for video layer with MDP composition

With video layer in MDP composition, there is no need to apply destination
over blend rule, as z-order of layers take care of proper staging of layers
by default. Apply constant blend to the video layer when in MDP composition

CRs-fixed: 437965
Change-Id: I7476659f12cdf06018bf6ace829a836c86ae97e8
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>