The software ring is created to support priority context while there is only
one hardware queue for gfx.

Every software ring has its fence driver and could be used as an ordinary ring
for the GPU scheduler.
Multiple software rings are bound to a real ring with the ring muxer. The
packages committed on the software ring are copied to the real ring.

v2: Use array to store software ring entry.
v3: Remove unnecessary prints.
v4: Remove amdgpu_ring_sw_init/fini functions,
using gtt for sw ring buffer for later dma copy
optimization.
v5: Allocate ring entry dynamically in the muxer.
v6: Update comments for the ring muxer.
v7: Modify for function naming.
v8: Combine software ring functions into amdgpu_ring_mux.c
v9: Use kernel-doc comment on the get_rptr function.

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>
Signed-off-by: Jiadong.Zhu <Jiadong.Zhu@amd.com>
Acked-by: Huang Rui <ray.huang@amd.com>
Acked-by: Luben Tuikov <luben.tuikov@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>