drm/radeon: fix sad_count check for dce3

Make it consistent with the sad code for other asics to deal
with monitors that don't report sads.

bug:
https://bugzilla.kernel.org/show_bug.cgi?id=89461



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