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

When the blended background pipe has dropped alpha and it is going to
be blended with foreground pipe on the next stage,
MDP4_BLEND_BG_ALPHA_FG_CONST is taken. Foreground alpha constant is
0xFF, so we need to use this alpha value to show background, instead
of its inverted value.

Change-Id: I9412141b28a46ed3b58079a56969e79dea4ed57c
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Naseer Ahmed <naseer@codeaurora.org>