drm/amdgpu: replace snprintf in show functions with sysfs_emit

show() must not use snprintf() when formatting the value to be
returned to user space.

Fix the following coccicheck warning:
drivers/gpu/drm/amd/amdgpu/amdgpu_ucode.c:427:
WARNING: use scnprintf or sprintf.

Signed-off-by: Qing Wang <wangqing@vivo.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>