drm/amd: Use the first non-dGPU PCI device for BW limits

When bandwidth limits are looked up using pcie_bandwidth_available()
virtual links such as USB4 are analyzed which might not represent the
real speed. Furthermore devices may change speeds autonomously which
may introduce conditional variation to the results reported in the
status registers.

Instead look at the capabilities of first PCI device outside of
dGPU to decide upper limits that the dGPU will work at.

For eGPU this effectively means that it will use the speed of the link
partner.

Link: https://gitlab.freedesktop.org/drm/amd/-/issues/2925#note_2145860
Link: https://www.usb.org/document-library/usb4r-specification-v20
      USB4 V2 with Errata and ECN through June 2023
      Section 11.2.1
Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>