msm: mdss: avoid danger/safe signal disabling for NRT clients

NRT clients need not to enable or disable danger/safe signalling.
So pipe used by non real time clients (e.g. writeback path) need
not enable or disable danger/safe signalling explicitly. Hence
add a check to ensure danger/safe signalling enabled only for
real time client at pipe_init and disabled at pipe_free.

Change-Id: I69b8b57620eb0c27193671632f9a97cccab073f3
Signed-off-by: Radhika Ranjan Soni <rrsoni@codeaurora.org>