drm/amdgpu: Adjust _PR3 detection

drm/amdgpu: Adjust _PR3 detection

_PR3 detection was changed in commit 134b8c5d8674 ("drm/amd: Fix
detection of _PR3 on the PCIe root port") to look at the root port
of the topology containing the GPU.  This however was too far because
it ignored whether or not all the intermediary bridges could power
off the device.  The original design in commit b10c1c5b3a4e ("drm/amdgpu:
add check for ACPI power resources") was too narrow because it matched
the switches internal to the GPU.

Use the goldilocks approach and look for the first bridge outside of the
GPU and check for _PR3 on that device.

Fixes: 134b8c5d8674 ("drm/amd: Fix detection of _PR3 on the PCIe root port")
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>