drm/amd: use drm_dbg_kms to log addfb2 failures

Avoid printing an error with dev_err, because user-space can trigger
these at will by issuing an ioctl.

Convert a DRM_DEBUG_KMS call to the more modern drm_dbg_kms macro.

Signed-off-by: Simon Ser <contact@emersion.fr>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Harry Wentland <hwentlan@amd.com>
Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Michel Dänzer <michel@daenzer.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>