drm/i915: Fix test on inputs for vma_compare()

drm/i915: Fix test on inputs for vma_compare()

When supplying a view to vma_compare() it is required that the supplied
i915_address_space is the global GTT. I tested the VMA instead (which is
the current position in the rbtree and maybe from any address space).

(This reapplies commit a44342acde30 ("drm/i915: Fix test on inputs for
vma_compare()") as it was lost in the vma split)

Reported-by: Matthew Auld <matthew.auld@intel.com>
Tested-by: Matthew Auld <matthew.auld@intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=98579
Fixes: db6c2b4151f2 ("drm/i915: Store the vma in an rbtree...")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161103200852.23431-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reported-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Fixes: b42fe9ca0a1e ("drm/i915: Split out i915_vma.c")
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>