msm: mdss: add atraces for tracking atomic commit

Add atraces tag to track atomic commit. This would be helpful
in analysing systraces to isolate any delays during validate
and commit cycle.

CRs-fixed: 930504
Change-Id: I204e620283926b03f3e956731145b99d81b29e6d
Signed-off-by: Veera Sundaram Sankaran <veeras@codeaurora.org>