drm/amdgpu: rework ctx entity creation

Use a fixed number of entities for each hardware IP.

The number of compute entities is reduced to four, SDMA keeps it two
entities and all other engines just expose one entity.

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