drm/i915: Remove vm arg from relocate entry

The only place we were using it was for GEN6, which won't have PPGTT
support anyway (ie. the VM is always the same). To clear things up,
(it only added confusion for me since it doesn't allow us to assert
vma->vm is what we always want, when just looking at the code).

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>