drm/msm/sde: fix flush bit setting in pp-split case

For single flush cases (dual-ctl or pp-split), skip
setting the flush bit for the slave interface, since
both interfaces use same ctl and HW will only flush
the master.

Change-Id: Ic48a1e0acbc1c8c215cbc41db5a3dc8daccbfa7d
Signed-off-by: Raviteja Tamatam <travitej@codeaurora.org>