drm/amdgpu: fix incorrect pcie_gen_mask in passthrough case

[why]
Passthrough case is treated as root bus and pcie_gen_mask is set as
default value that does not support GEN 3 and GEN 4 for PCIe link
speed. So PCIe link speed will be downgraded at smu hw init in
passthrough condition

[how]
Move get pci info after detect virtualization and check if it is
passthrough case when set pcie_gen_mask

Signed-off-by: Tong Liu01 <Tong.Liu01@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>