libdrm: Use DRM_IOCTL_GET_PCIINFO on FreeBSD
This has only landed in FreeBSD's drm in February 2021, so keep the old code (sysctl and /dev/pci) as fallback, at least for now.
I've been running this patch since then, and I have tested it once with older drm (without the pciinfo changes), but more testing is welcome.