msm: ext-disp: check for NULL before accessing codec ops

In case audio codec driver has failed to register with ext-disp
module, cable connect or disconnect request from DisplayPort driver
can still be processed to update the internal states. Check for
codec ops before accessing it during disconnect as it might be
NULL.

CRs-Fixed: 2128730
Change-Id: I917f0670228da1ebb3fc53064c4844c3f2a57d61
Signed-off-by: Ajay Singh Parmar <aparmar@codeaurora.org>