drm/msm/sde: clear blend stages of all mixers

Currently the blending stages of all the mixers is not
cleared when switching mixer configurations or configuring
solid fill mode.

As a result stale pipes remain connected when switching
mixer configurations causing smmu faults.

Fix the logic to clear the blend stages of all the mixers.

Change-Id: I1a3c5a020d6bc04657cb5357a2cd60c1a53ce4c7
Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>