drm/msm/sde: enable proper support for split flush

Enable atomic hardware flushing of two data paths via a single
flush register. Previously, different data paths were flushed
separately from software, which could potentially result in
the flushes happening on different frames.

Change-Id: I858a0f737743d48404f47af85b407e1cddf0a0a7
Signed-off-by: Clarence Ip <cip@codeaurora.org>