drm/i915/audio: port is going to be just fine, simplify checks

If it was wrong, we'd be screwed already.

Cc: Libin Yang <libin.yang@linux.intel.com>
Reviewed-by: Libin Yang <libin.yang@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/8cf454ccefc05b234aa81c45a4ce9018e7c9324f.1476111629.git.jani.nikula@intel.com