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>
diff --git a/drivers/gpu/drm/i915/i915_gem_gtt.c b/drivers/gpu/drm/i915/i915_gem_gtt.c
index 76dd3b4..6ffa12b 100644
--- a/drivers/gpu/drm/i915/i915_gem_gtt.c
+++ b/drivers/gpu/drm/i915/i915_gem_gtt.c
@@ -1180,7 +1180,7 @@
 	return 0;
 }
 
-int i915_gem_init_ppgtt(struct drm_device *dev, struct i915_hw_ppgtt *ppgtt)
+int i915_ppgtt_init(struct drm_device *dev, struct i915_hw_ppgtt *ppgtt)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	int ret = 0;
@@ -1211,6 +1211,19 @@
 	return ret;
 }
 
+void  i915_ppgtt_release(struct kref *kref)
+{
+	struct i915_hw_ppgtt *ppgtt =
+		container_of(kref, struct i915_hw_ppgtt, ref);
+
+	/* vmas should already be unbound */
+	WARN_ON(!list_empty(&ppgtt->base.active_list));
+	WARN_ON(!list_empty(&ppgtt->base.inactive_list));
+
+	ppgtt->base.cleanup(&ppgtt->base);
+	kfree(ppgtt);
+}
+
 static void
 ppgtt_bind_vma(struct i915_vma *vma,
 	       enum i915_cache_level cache_level,
@@ -2148,15 +2161,12 @@
 				  struct i915_address_space *vm)
 {
 	struct i915_vma *vma;
-	struct i915_hw_ppgtt *ppgtt = NULL;
 
 	vma = i915_gem_obj_to_vma(obj, vm);
 	if (!vma)
 		vma = __i915_gem_vma_create(obj, vm);
 
-	ppgtt = vm_to_ppgtt(vm);
-	if (ppgtt)
-		kref_get(&ppgtt->ref);
+	i915_ppgtt_get(vm_to_ppgtt(vm));
 
 	return vma;
 }