drm/amdgpu: improve error handling in pcie_bw

1. Initialize the counters to 0 in case the callback
   fails to initialize them.
2. The counters don't exist on APUs so return an error
   for them.
3. Return an error if the callback doesn't exist.

Reviewed-by: Yong Zhao <Yong.Zhao@amd.com>
Reviewed-By: Kent Russell <kent.russell@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>