drm/amdgpu: add entity only when first job come

umd somtimes will create a context for every ring,
that means some entities wouldn't be used at all.

Signed-off-by: Chunming Zhou <David1.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>