drm/amdgpu: add CE preamble flag v3

The CE preamble IB can be dropped for the same context

v2: use the flags directly
v3: remove 'CE' for potential preamble usage by other rings

Signed-off-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>