msm: mdss: Reset right mixer flush bits in split-display use-case

Split-display uses left and right mixers in a single control path. In
such scenarios, flush bits variable holding right mixer updates needs
to be reset in along with left mixer.

Change-Id: I66060b7be5a38e2c08e0f1d3d82a18062e2bd8c8
Signed-off-by: Ujwal Patel <ujwalp@codeaurora.org>