drm/msm/sde: reset mixers swapped state on acquiring mixers

Reset the mixers swapped status when setting up the mixers at
the start and end a use case. This addresses the case where the
last frame before disable was a right-only partial update
requiring a mixer swap due to DSC merge topology.

Change-Id: I1af2480886ab72ef689bc1d37d890bf3f4e135c5
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>