drm/amd/amdgpu: Support full range of GFX ring names

Right now there's only one but the rest of the code is being
setup to support more so might as well fix this up too.

Signed-off-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>