drm/amdgpu: Add software ring callbacks for gfx9 (v8)

Set ring functions with software ring callbacks on gfx9.

The software ring could be tested by debugfs_test_ib case.

v2: Set sw_ring 2 to enable software ring by default.
v3: Remove the parameter for software ring enablement.
v4: Use amdgpu_ring_init/fini for software rings.
v5: Update for code format. Fix conflict.
v6: Remove unnecessary checks and enable software ring on gfx9 by default.
v7: Use static array for software ring names and priorities.
v8: Stop creating software rings if no gfx ring existed.

Cc: Christian Koenig <Christian.Koenig@amd.com>
Cc: Luben Tuikov <Luben.Tuikov@amd.com>
Cc: Andrey Grodzovsky <Andrey.Grodzovsky@amd.com>
Cc: Michel Dänzer <michel@daenzer.net>
Cc: Likun Gao <Likun.Gao@amd.com>
Signed-off-by: Jiadong.Zhu <Jiadong.Zhu@amd.com>
Acked-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>