drm/msm/dp: validate audio ops in cable disconnect path

There is possiblity for accessing invalid audio ops when
hpd unplug is processed in between dp_display_enable
and dp_display_post_enable functions. This change adds
the validation for audio ops before accessing them and
clears audio_supported variable after processing
dp_audio_off functionality to initialize the variable to
correct state.

CRs-Fixed: 2243808
Change-Id: I103329ec5a922e5a16ce45388b3c09165b97b757
Signed-off-by: Govinda Rajulu Chenna <gchenna@codeaurora.org>