drm/amdgpu/ttm: Consider concurrent VM flushes for buffer entities

drm/amdgpu/ttm: Consider concurrent VM flushes for buffer entities

Allow using multiple SDMA schedulers only on GPUs where
we are allowed to do concurrent VM flushes.
This consideration is necessary because all GART windows
are mapped in VMID 0 (the kernel VMID) so each buffer
entity would flush VMID 0 concurrently.

Practically this means that we can't use multiple SDMA
engines for TTM on GFX6-8 and Navi 1x.

Fixes: 01c836788b37 ("drm/amdgpu: pass all the sdma scheds to amdgpu_mman")
Fixes: e4029f7a9474 ("drm/amdgpu: only use working sdma schedulers for ttm")
Cc: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>