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>