Revert "msm_fb: display: remove background invert alpha option for alpha drop pipe"

This reverts commit 0119007a18aae8a8fb43f4320509bda161da4167.

CRs-fixed: 461574
Change-Id: Ia768030ec859a4ff38549686936789413826df32
Signed-off-by: Mayank Chopra <makchopra@codeaurora.org>
msm: display: plane alpha support

Clean up the blending rule to follow blend_op set from hwc.
Use modulate alpha for plane alpha when it is not 0xff.
Keep backward compatibility if blend_op is not set.

Change-Id: I02fd3c3c7b5ace2e6eec22b2db0284161404a0fc
Signed-off-by: Ken Zhang <kenz@codeaurora.org>
Merge changes I02fd3c3c,Ia768030e into jb-mr1

* changes:
  msm: display: plane alpha support
  Revert "msm_fb: display: remove background invert alpha option for alpha drop pipe"