drm/i915: Fix a typo in vgt_balloon_space()

Commit 625d988 ("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: 625d988 ("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>

drivers/gpu/drm/i915/i915_vgpu.c

@@ -122,7 +122,7 @@ static int vgt_balloon_space(struct i915_ggtt *ggtt,
 {
 	unsigned long size = end - start;
 
-	if (start <= end)
+	if (start >= end)
 		return -EINVAL;
 
 	DRM_INFO("balloon space: range [ 0x%lx - 0x%lx ] %lu KiB.\n",