drm/i915: Some cleanups for the ppgtt lifetime handling

So when reviewing Michel's patch I've noticed a few things and cleaned
them up:
- The early checks in ppgtt_release are now redundant: The inactive
  list should always be empty now, so we can ditch these checks. Even
  for the aliasing ppgtt (though that's a different confusion) since
  we tear that down after all the objects are gone.
- The ppgtt handling functions are splattered all over. Consolidate
  them in i915_gem_gtt.c, give them OCD prefixes and add wrappers for
  get/put.
- There was a bit a confusion in ppgtt_release about whether it cares
  about the active or inactive list. It should care about them both,
  so augment the WARNINGs to check for both.

There's still create_vm_for_ctx left to do, put that is blocked on the
removal of ppgtt->ctx. Once that's done we can rename it to
i915_ppgtt_create and move it to its siblings for handling ppgtts.

v2: Move the ppgtt checks into the inline get/put functions as
suggested by Chris.

v3: Inline the now redundant ppgtt local variable.

Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -4477,7 +4477,6 @@ struct i915_vma *i915_gem_obj_to_vma(struct drm_i915_gem_object *obj,
 void i915_gem_vma_destroy(struct i915_vma *vma)
 {
 	struct i915_address_space *vm = NULL;
-	struct i915_hw_ppgtt *ppgtt = NULL;
 	WARN_ON(vma->node.allocated);
 
 	/* Keep the vma as a placeholder in the execbuffer reservation lists */
@@ -4485,10 +4484,8 @@ void i915_gem_vma_destroy(struct i915_vma *vma)
 		return;
 
 	vm = vma->vm;
-	ppgtt = vm_to_ppgtt(vm);
 
-	if (ppgtt)
-		kref_put(&ppgtt->ref, ppgtt_release);
+	i915_ppgtt_put(vm_to_ppgtt(vm));
 
 	list_del(&vma->vma_link);