drm/amdgpu: Fix null pointer amdgpu_device_fw_loading

Need to check adev->powerplay.pp_funcs.

Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>