Mako: Revert msm_fb: remove background invert alpha option for alpha drop pipe

This fixes the pie controls visual glitch when launching apps on the mako and
is required for this http://review.cyanogenmod.org/#/c/35314/ to work.

This reverts: f4f9d28eac1024146c8b074742fcb6793523eb54
  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.
  Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>

Change-Id: Ie70d1f27dae2751a1eea98fd145145d9b40f3d1d