drm/amdgpu: use separate status for buffer funcs availability v2

The ring status can change during GPU reset, but we still need to be
able to schedule TTM buffer moves in the meantime.

Otherwise we can ran into problems because of aborted move/fill
operations during GPU resets.

v2: still check if ring is available during direct submit.

Signed-off-by: Christian König <christian.koenig@amd.com>
Acked-by: Chunming zhou <david1.zhou@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>