drm/i915/display: update intel_dp_has_audio to support MST

Modify intel_dp_has_audio to handle DP-MST as well.

v1: fix the wrong port comparison (Jani Nikula)

Signed-off-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230822204818.109742-2-vinod.govindapillai@intel.com