drm/amdgpu: check correct allocated mqd_backup object after alloc

Instead of the default one, check the right mqd_backup object.

Signed-off-by: Guchun Chen <guchun.chen@amd.com>
Cc: Le Ma <le.ma@amd.com>
Reviewed-by: Le Ma <le.ma@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>