drm/amdgpu: fix num_rbs exposed to userspace (v2)

This was accidently broken for harvest cards when the
code was refactored for Polaris support.

v2: multiply by shader engines.  Noticed by Nicolai.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org