drm/i915: Correct discard i915_vma_compare assertion

As a last minute addition, I added an assertion to make sure that the
new i915_vma view would be equal to the discard. However, the positive
encouragement from CI only goes to show that we rarely take this path,
and it wasn't until the post-merge run did we hit the assert -- because
it compared the wrong view. Fixup the copy'n'paste error and compare
against both the old view and the expected new view.

Fixes: 9bdcaa5e

 ("drm/i915: Discard a misplaced GGTT vma")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200605184844.24644-1-chris@chris-wilson.co.uk