drm/amd/pm: fix NULL pointer issue of amdgpu_smu_stb_debug_fs_init

Fix NULL pointer issue on amdgpu_smu_stb_debug_fs_init if SMU block not
enabled.

Signed-off-by: Likun Gao <Likun.Gao@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>