drm/msm/sde: reset ctl mixer config at end of use case

Need to take the control path out of any source split or other
configuration at the end of a use case to prepare for the next
client.

Change-Id: I9a2fb57dcafc576a23a13da1906a44e9992aeb74
Signed-off-by: Lloyd Atkinson <latkinso@codeaurora.org>