msm: mdss: dp: do not program audio registers when DP core is off

It is possible that the DP core may already be shutdown when a
callback from the audio subsystem is invoked. In such cases, ensure
that the callbacks return gracefully without trying to do any
hardware programming.

CRs-Fixed: 2072592
Change-Id: Ib112c0791852e1c835c739167683f160723b3baa
Signed-off-by: Aravind Venkateswaran <aravindh@codeaurora.org>