drm/amdgpu: correct the return value for error case

It should not return 0 for error case as '0' is actually
a special value for index.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>