drm/amdgpu/gfx8: move CE&DE meta data structure to vi_structs.h

Because different HWs have different definition for CE & DE meta
data, follow mqd design to move the structures to vi_structs.h.

And change the prefix from amdgpu to vi as the structures is only
for VI family.

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