drm/msm/dpu: check split role for single flush

Removing unwanted access of crtc_state for finding this information.
Use split role information to know whether we have slave ctl.

Signed-off-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1550107156-17625-8-git-send-email-jsanka@codeaurora.org


Signed-off-by: Rob Clark <robdclark@chromium.org>