agp/intel: split out gmch/gtt probe, part 1

This is essentially the last piece of code that's tying intel-gtt.c
to intel-agp.c. Extract the probe code into it's own function so that
it can be moved to intel-gtt.c.

This requires some slight changes in the ordering of device probe function.
This patch just implements that for better bisectability in case this
introduces bugs.

The biggest change is that the gmch/gtt code doesn't execute a pci
resource fixup anymore. I've dug around in historical git trees, and
this change is to support the agp port on an old HP server with the
i440 intel chipset. So only needed for the agp driver.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Eric Anholt <eric@anholt.net>