bhyve: Conditionally compile framebuffer-related code

The arm64 port does not implement VGA, so move the device model sources.
Compile framebuffer code only on amd64 for now, but do not move the
sources, as we ought to be able to add support later.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40740