amdgpu: add error return value for finding bo by cpu mapping (v2)

If nothing is found, error should be returned.

v2: udpate the error value different from parameter check

Signed-off-by: Junwei Zhang <Jerry.Zhang@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>