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

include/uapi/drm/amdgpu_drm.h

@@ -445,7 +445,7 @@ union drm_amdgpu_cs {
 /* This IB should be submitted to CE */
 #define AMDGPU_IB_FLAG_CE	(1<<0)
 
-/* CE Preamble */
+/* Preamble flag, which means the IB could be dropped if no context switch */
 #define AMDGPU_IB_FLAG_PREAMBLE (1<<1)
 
 struct drm_amdgpu_cs_chunk_ib {