uapi/drm:change Preamble Description

Preamble in linux doesn't mean it is CE PREAMBLE IB,
instead it means this IB could be dropped if no
ctx switch happens.

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