To allow a smooth transition from pinning buffer objects to dynamic
invalidation we first start to cache the sg_table for an attachment.

v2: keep closer to the DRM implementation
Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.kernel.org/patch/10943053/