drm/amdgpu: drop navi pcie bw callback

It's not implemented yet so just drop it so the sysfs
pcie bw file returns an appropriate error instead of
garbage.

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>