drm/i915: use correct node for handling cache domain eviction

It looks like we were incorrectly comparing vma->node against itself
instead of the target node, when evicting for a node on systems where we
need guard pages between regions with different cache domains. As a
consequence we can end up trying to needlessly evict neighbouring nodes,
even if they have the same cache domain, and if they were pinned we
would fail the eviction.

Fixes: 625d988a

 ("drm/i915: Extract reserving space in the GTT to a helper")
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170306235414.23407-3-matthew.auld@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>