Solved
lspci -nnk
01:00.0 Multimedia video controller [0400]: Conexant Systems, Inc. CX23887/8
however
lsmod | grep cx
does not report cx23887 drivers, but does report cx23885
so, add the following to proxmox /etc/modprobe.d/blacklist.conf:
blacklist cx23885
and I can now passthrough the pcie...