Fix scfb to only attach the device if it can drive it.

[rezny]

Summary

This is a fix to allow fallback to vesa to work correctly. scfb is probed before vesa so that it will be used on PCs with UEFI, but scfb would always claim the device during Probe so vesa sees no device during Probe, meaning the X server bails on PCs with traditional BIOS instead oF using vesa. This patch adds a check for a framebuffer in Probe (copied from PreInit) before claiming the PCI device so. Also, assign the port to x11@ after discussion with ray@ on IRC. x11@ should take over maintenance of scfb itself as well.

Test Plan

verified fallback to vesa now works on 11 amd64, verified scfb (using ofwfb) still works on 11 ppc64

[swills]

Approved