drm/amdgpu: only check for _PR3 on dGPUs

We don't support runtime pm on APUs.  They support more
dynamic power savings using clock and powergating.

Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Tested-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>