drm/amdgpu: give ttm entities access to all the sdma scheds

drm/amdgpu: give ttm entities access to all the sdma scheds

With this change we now have as many clear and move entities as we
have sdma engines (limited to TTM_NUM_MOVE_FENCES).

To enable load-balancing this patch gives access to all sdma
schedulers to all entities, except default_entity which has no
use for multiple schedulers.

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v6: add comment
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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>