drm/radeon/audio: make sure connector is valid in hotplug case

Avoids a crash when a monitor is hotplugged and the encoder
and connector are not linked yet.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=90681



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org