drm/i915/bdw: Only use 2g GGTT for 32b platforms

Daniel requested in the bug that I use a 3GB fallback size. Since this
is not in the spec as a valid size, I decided against it. We could
potentially add a patch to bump it to 3GB on top of this one.

This probably should be CC: stable - but I'll let the powers that be
decide that one.

Regression from a revert of the revert:
commit 7907f45b


Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Wed Feb 19 22:05:46 2014 -0800

    Revert "drm/i915/bdw: Limit GTT to 2GB"

v2: Change ifdef to 32b, instead of ifndef
update comment

v3. Update comment to not wrap (Daniel).
Update commit message

v4: s/CONFIG_32/CONFIG_X86_32 (Jani).

v5: s/CONFIG_x86_32BIT/CONFIG_x86_32, as meant in v4
s/32B/32b (chris)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76619
Cc: stable@vger.kernel.org
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Tested-by: "Yang, Guang A" <guang.a.yang@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>