drm/msm/sde: fix wrong alpha inverse in coverage blend op

For the 'coverage' blend op, the alpha in the
foreground side of the alpha blending equation
should not be inverse. However, current code
uses the inverse of the foreground alpha when
setting the 'coverage' blend op flags.
Fix this problem by removing the inverse in the
foreground for the 'coverage' alpha blending op.

Change-Id: Iabefe3f4e5bf01ca0da664c85e5618ab7017259f
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>