the installation kernel knows of nvidia, however the p9 distribution
you installed does not..  the message 'unknown controller' is
indicative of aux/vga (the new one you installed) not having nvidia
support..

download the vganvidia.c, nvidia.c and something.h files and do the
following:

just add the nvidia.c and registers.h (forgot the exact name) to
/sys/src/cmd/aux/vga/, edit the mkfile to get nvidia.c to compile,
edit data.c to add nvidia and nvidiahwgc controllers (the Ctlr*
ctlrs[] at the beginning) and you're almost set..  type 'mk install'
in the same directory (disk/kfscmd allow may be required) and you'll
have an aux/vga that speaks nvidia..  restart and mail me privately if
it's still not working (you may have to add support in the kernel too
-- the vganvidia.c file is for that)...

andrey