drm/radeon: fix null pointer dereference in dce6+ audio code

Don't crash if the encoder does not have an afmt struct.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=72283

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