drm/amdgpu: Check num_gfx_rings for gfx v9_0 rb setup.

No need to set up rb when no gfx rings.

Signed-off-by: Candice Li <candice.li@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>