Revert "drm/i915: Fix detection of base of stolen memory"

This reverts commit 53e587aa5ca81497d0ea6e340320ec5778d1f311.
Yves-Alexis Perez reported that it broke the driver on two machines
with GMA4500 and i965 chips.  Only the backported version in 3.2.45
had this effect; the fix in mainline is fine.

Daniel Vetter stated that the fix is not needed in 3.2 anyway.

Reported-by: Yves-Alexis Perez <corsac@debian.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>