drm/i915: Don't try to tear down the stolen drm_mm if it's not there

Every other place properly checks whether we've managed to set
up the stolen allocator at boot-up properly, with the exception
of the cleanup code. Which results in an ugly

*ERROR* Memory manager not clean. Delaying takedown

at module unload time since the drm_mm isn't initialized at all.

v2: While at it check whether the stolen drm_mm is initialized instead
of the more obscure stolen_base == 0 check.

v3: Fix up the logic. Also we need to keep the stolen_base check in
i915_gem_object_create_stolen_for_preallocated since that can be
called before stolen memory is fully set up. Spotted by Chris Wilson.

v4: Readd the conversion in i915_gem_object_create_stolen_for_preallocated,
the check is for the dev_priv->mm.gtt_space drm_mm, the stolen
allocatot must already be initialized when calling that function (if
we indeed have stolen memory).

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65953
Cc: Chris Wilson <[e-mail]>
Tested-by: lu hua <[e-mail]> (v3)
Reviewed-by: Chris Wilson <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/i915/i915_gem_stolen.c

···
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
 
-	if (dev_priv->mm.stolen_base == 0)
+	if (!drm_mm_initialized(&dev_priv->mm.stolen))
 		return -ENODEV;
 
 	if (size < dev_priv->cfb_size)
···
 void i915_gem_cleanup_stolen(struct drm_device *dev)
 {
 	struct drm_i915_private *dev_priv = dev->dev_private;
+
+	if (!drm_mm_initialized(&dev_priv->mm.stolen))
+		return;
 
 	i915_gem_stolen_cleanup_compression(dev);
 	drm_mm_takedown(&dev_priv->mm.stolen);
···
 	struct drm_i915_gem_object *obj;
 	struct drm_mm_node *stolen;
 
-	if (dev_priv->mm.stolen_base == 0)
+	if (!drm_mm_initialized(&dev_priv->mm.stolen))
 		return NULL;
 
 	DRM_DEBUG_KMS("creating stolen object: size=%x\n", size);
···
 	struct drm_i915_gem_object *obj;
 	struct drm_mm_node *stolen;
 
-	if (dev_priv->mm.stolen_base == 0)
+	if (!drm_mm_initialized(&dev_priv->mm.stolen))
 		return NULL;
 
 	DRM_DEBUG_KMS("creating preallocated stolen object: stolen_offset=%x, gtt_offset=%x, size=%x\n",