drm/msm: dpu: Add tracing around CTL_FLUSH

I found these tracepoints useful for debugging cursor/ctl, someone else
might find them useful too

Reviewed-by: Jeykumar Sankaran <jsanka@codeaurora.org>
Reviewed-by: Abhinav Kumar <abhinavk@codeaurora.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Rob Clark <robdclark@gmail.com>