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>

drivers/gpu/drm/amd/pm/swsmu/amdgpu_smu.c

@@ -3031,7 +3031,7 @@ void amdgpu_smu_stb_debug_fs_init(struct amdgpu_device *adev)
 
 	struct smu_context *smu = adev->powerplay.pp_handle;
 
-	if (!smu->stb_context.stb_buf_size)
+	if (!smu || (!smu->stb_context.stb_buf_size))
 		return;
 
 	debugfs_create_file_size("amdgpu_smu_stb_dump",