uapi/drm:add new flag for Preemption

when MCBP supported, we will set pre_enb bit for those
IBs with PREEMPT flag tagged

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>