msm_fb: display: fix mdp blending logic

Handle case where blending downscaled VG pipe with alpha channel since
alpha channel will be dropped by MDP hw. Also propagate framebuffer pipe
alpha to top layers only if they are opaque to avoid picking up from
layers which contain alpha.

CRs-Fixed: 326178
Change-Id: I73cfc0716d1f5f8a803a645bdc09b97f909e593d
Signed-off-by: Adrian Salido-Moreno <adrianm@codeaurora.org>