Modified driver / device probing

[Ian Romanick]

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Here are links to two patches.  The first patch implements a new driver
/ device probing mechanism in the server, and the second implements the
new entry-point in the mga driver.

http://users.freedesktop.org/~idr/patches/new_probe-xserver-200602221648.patch
http://users.freedesktop.org/~idr/patches/new_probe-mga-200602221648.patch

The basic change is that the DriverRec now contains the list of PCI
devices supported by the driver.  The server uses that list in
conjunction with the list of Device sections and the list of available
PCI devices to determine which devices match the driver.  When a
matching device is found, the new PciProbe function is called.  This
just refactors existing code from the driver up to the server.

I do *not* intend to commit this code.  This is just a mock up of a
mechanism that could be used in conjunction with the new PCI code that
I'm writing.  The chipsets, pci_chipsets, and pci_vendor_id fields would
be replace with an array of pci_match structures:

struct pci_match {
	uint32_t  device, vendor;
	uint32_t  subdevice, subvendor;
	uint32_t  class, class_mask;
	intptr_t  driver_data;
};

Yes, it is intentional that pci_match is identical to the Linux kernel's
pci_device_id structure.  It is intended to be used in exactly the same
way by drivers.

In working on these changes I noticed a couple things that are worth
pointing out.

1. This version of this technique will break "umbrella" drivers like
ati.  It is possible to use pci_match::driver_data to determine which
real PciProbe function to call, but I don't see a clean way to implement
that until then.

2. Like the modified mga driver, most driver will only need a PciProbe
function.  Some drivers, especially those which support other buses,
will still need a Probe function.  However, it should be possible to use
the existing Probe function with little or no modification in those cases.

3. Why does ScreenInfoPtr have a Probe function?
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