drm/i915: Fix a typo in vgt_balloon_space()

drm/i915: Fix a typo in vgt_balloon_space()

Commit 625d988acc28 ("drm/i915: Extract reserving space in the GTT to a
helper") introduces this typo which can cause a driver loading failure
in Linux GVT-g guest.

Fixes: 625d988acc28 ("drm/i915: Extract reserving space in the GTT to a helper")
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Zhiyuan Lv <zhiyuan.lv@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1484661972-9366-1-git-send-email-zhi.a.wang@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>