bhyve: assign a valid INTPIN to NVIDIA GPUs

When passing an NVIDIA GPU to a Linux VM, the GPU driver refuses to work and
complains about a missing IRQ:

[ 77.208984] NVRM: Can't find an IRQ for your NVIDIA card!
[ 77.212697] NVRM: Please check your BIOS settings.
[ 77.212699] NVRM: [Plug & Play OS] should be set to NO
[ 77.212700] NVRM: [Assign IRQ to VGA] should be set to YES
[ 77.212702] nvidia: probe of 0000:00:07.0 failed with error -1

By setting a valid INTPIN in the PCI config space those error messages
disappear. Additionally, tools like nvidia-smi are able to detect the GPU and
the GPU driver seems to work properly.

Note that bhyve still doesn't support legacy interrupts. So, the guest
shouldn't try to use it even though we're assigning an INTPIN.

Reviewed by: jhb
Tested by: chuck
MFC after: 1 week
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D51892