drm/amdgpu/ring: simplify scheduler setup logic

Set up a GPU scheduler based on the ring flag rather
than the ring type.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>