drm/i915: Move vma vfuns to adddress_space

They change with the address space and not with each vma, so move them
into the right pile of vfuncs. Save 2 pointers per vma and clarifies
the code.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 761f1b6..8698cff 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -3067,7 +3067,7 @@
 
 	trace_i915_vma_unbind(vma);
 
-	vma->unbind_vma(vma);
+	vma->vm->unbind_vma(vma);
 
 	list_del_init(&vma->mm_list);
 	if (i915_is_ggtt(vma->vm)) {