drm/amdgpu: use sysfs_emit() to instead of scnprintf()

Replace the open-code with sysfs_emit() to simplify the code.

Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Signed-off-by: ye xingchen <ye.xingchen@zte.com.cn>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>