drm/radeon: initialize sadb to NULL in the audio code

Fixes kfree of the sadb buffer when it's NULL.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

diff --cc drivers/gpu/drm/radeon/dce3_1_afmt.c
Simple merge

diff --cc drivers/gpu/drm/radeon/dce6_afmt.c
Simple merge

diff --cc drivers/gpu/drm/radeon/evergreen_hdmi.c
Simple merge