drm/amdgpu: Fix null point error

need to check adev->powerplay.pp_funcs first, becasue from
AI, the smu ip can be disabled by user, and the pp_handle
is null in this case.

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>