[3.4.y, 3.5.y] drm/i915: Use the correct size of the GTT for placing the per-process entries

[Jonathan Nieder]

Hi Greg,

Please consider

 9a0f938bde74 drm/i915: Use the correct size of the GTT for placing
              the per-process entries, 2012-08-24

for application to the 3.4.y tree.

Without this patch, Geoff Crompton's iMac hits a BUG during bootup.
The problem is reproducible on

 * Debian's 3.2.y-based kernel with drm backported from 3.4.37
 * a Debian kernel close to 3.4.4
 * a Debian kernel close to 3.5.5
 * vanilla 3.4.4

He is not able to reproduce the problem on

 * Debian's older 3.2.y-based kernels without the 3.4.y drm backport
 * a Debian kernel close to 3.6.4; various newer kernels
 * vanilla 3.4.4 + this patch

The patch was applied upstream during the 3.6-rc4 cycle, so newer
kernels don't need it.

http://bugs.debian.org/703468 has details, including a screenshot of
the boot failure ("unable to handling kernel paging request at
ffffc9000b7ff000" in i915_gem_init_ppgtt).

Thoughts welcome, as always.

Regards,
Jonathan