hostb(4) and vgapci(4) patch

I have a patch that is an attempt to untangle a few things in relation to 
Host-PCI bridges and VGA PCI devices.  Basically, the change is to create a 
more "real" hostb driver as well as a new vgapci driver and to change agp, 
drm, and acpi_video to attach to these drivers.  This means among other 
things:

- In theory you can now kldload agp after boot since it still has a place to 
attach to.
- i830/915 drm is no longer a child of agp, instead both become children of 
vgapci0.
- You can now use acpi_video with drm as both attach as children of vgapci0.
- This provides a way for us to possibly solve the DPMS problem for 
suspend/resume (including a cleaner way to do the hack dpms patch I posted to 
acpi_at_ a long while ago that several people still use).

Some other details include:

- agp devices no longer map the _entire_ aperture into contiguous KVA meaning 
that it might be possible now to use a 256 MB aperture without panicing
- I've added a new pci_if.m method for locating a specific capability for a 
PCI device.

I have tested this on my laptop and verified that dri still works, but it 
needs some wider testing, especially the i830/i915 case is slightly more 
complicated.  Also, this is not going to work with the nvidia-driver 
currently, but that's something that can be fixed in the future.  If the agp 
non-mapping does fix the 256 MB aperture issues then I will probably MFC that 
part to RELENG_6.

http://www.FreeBSD.org/~jhb/patches/agp_cvs.patch

-- 
John Baldwin <jhb_at_FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve"  =  http://www.FreeBSD.org