msm: mdss: Remove background blending config for FB

Background blending for framebuffer isn't handled specially
in the driver. All blending assumes source over destination blending
from userspace.

CRs-fixed: 470535
Change-Id: Idbff01d9d976e57daaa2e5c128575a0dec07b38b
Signed-off-by: Sree Sesha Aravind Vadrevu <svadrevu@codeaurora.org>