drm/amdgpu: only use working sdma schedulers for ttm

drm/amdgpu: only use working sdma schedulers for ttm

It's possible that some sdma instances aren't working so we shouldn't try
to use them from TTM. To achieve this, delay the call to
amdgpu_sdma_set_buffer_funcs_scheds after the rings have been tested, and
then use the 'ready' property to decide if a sched should be used or not.

Note that currently it's not doing much, because if the ring helper fails
for any ring, the whole sdma block init fails.

---
v5: check buffer_funcs_enabled from amdgpu_ttm_access_memory_sdma
---

Signed-off-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>