drm/radeon: be quiet when no SAD block is found

It is fine for displays without audio functionality to not provide
any SAD block in their EDID. Do not log an error in that case,
just return quietly.

Inspired by a similar fix to the amdgpu driver in the context of bug
fdo#107825:
https://bugs.freedesktop.org/show_bug.cgi?id=107825



Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: "Christian König" <christian.koenig@amd.com>
Cc: "David (ChunMing) Zhou" <David1.Zhou@amd.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>