drm/amdgpu: return the PCIe gen and lanes from the INFO ioctl

For computing PCIe bandwidth in userspace and troubleshooting PCIe
bandwidth issues. Note that this intentionally fills holes and padding
in drm_amdgpu_info_device.

Mesa MR: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20790

Signed-off-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>