drm/msm/sde: enable single flush for video mode panels

For dual-dsi panels in video mode that use dual
controllers, enable single flush to make sure the
flush of both control paths is atomic in the HW.
This fixes race conditions between HW and SW
where if the SW flushes the registers independently
for each controller, the configuration can be
taken by the HW on different vsyncs, leading to
a misconfiguration and hence underruns  until next
flush is taken by HW.

Change-Id: I0c349c39bc172d9a0e11b45172b7dcfb7df47ba0
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>