agp/intel: Use the correct mask to detect i830 aperture size.

BugLink: https://bugs.launchpad.net/bugs/597075

commit f1befe71 introduced a
regression when detecting aperture size of some i915 adapters, e.g.,
those on the Intel Q35 chipset.

The original report: https://bugzilla.kernel.org/show_bug.cgi?id=15733
The regression report: https://bugzilla.kernel.org/show_bug.cgi?id=16294

According to the specification found at
http://intellinuxgraphics.org/VOL_1_graphics_core.pdf

, the PCI config
space register I830_GMCH_CTRL is a mirror of GMCH Graphics
Control. The correct macro for isolating the aperture size bits is
therefore I830_GMCH_GMS_MASK along with the attendant changes to the
case statement.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Tested-by: Kees Cook <kees.cook@canonical.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Eric Anholt <eric@anholt.net>
Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Eric Anholt <eric@anholt.net>