drm/i915/gtt: Rename i915_hw_ppgtt base member

In the near future, I want to subclass gen6_hw_ppgtt as it contains a
few specialised members and I wish to add more. To avoid the ugliness of
using ppgtt->base.base, rename the i915_hw_ppgtt base member
(i915_address_space) as vm, which is our common shorthand for an
i915_address_space local.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180605153758.18422-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_vma.c

@@ -85,7 +85,7 @@ vma_create(struct drm_i915_gem_object *obj,
 	int i;
 
 	/* The aliasing_ppgtt should never be used directly! */
-	GEM_BUG_ON(vm == &vm->i915->mm.aliasing_ppgtt->base);
+	GEM_BUG_ON(vm == &vm->i915->mm.aliasing_ppgtt->vm);
 
 	vma = kmem_cache_zalloc(vm->i915->vmas, GFP_KERNEL);
 	if (vma == NULL)