drm/i915: Reduce a pin-leak BUG into a WARN

commit 7e81a42e upstream.

Pin-leaks persist and we get the perennial bug reports of machine
lockups to the BUG_ON(pin_count==MAX). If we instead loudly report that
the object cannot be pinned at that time it should prevent the driver from
locking up, and hopefully restore a semblance of working whilst still
leaving us a OOPS to debug.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

drivers/gpu/drm/i915/i915_gem.c

@@ -3357,7 +3357,8 @@ i915_gem_object_pin(struct drm_i915_gem_object *obj,
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	int ret;
 
-	BUG_ON(obj->pin_count == DRM_I915_GEM_OBJECT_MAX_PIN_COUNT);
+	if (WARN_ON(obj->pin_count == DRM_I915_GEM_OBJECT_MAX_PIN_COUNT))
+		return -EBUSY;
 	WARN_ON(i915_verify_lists(dev));
 
 	if (obj->gtt_space != NULL) {