drm/i915: Don't BUG_ON in i915_gem_obj_offset

A WARN_ON is perfectly fine.

The BUG in here seems to be the cause behind hard-hangs when I cat the
i915_gem_pageflip debugfs file (which calls this from an irq
spinlock). But only while running a full igt run after a while. I
still need to root cause the underlying issue.

I'll also start reject patches which add new BUG_ON but don't come
with a really good justification for it. The general rule really
should be to just WARN and hope the driver survives for long enough.

v2: Make the WARN a bit more useful per Chris' suggestion.
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -5078,12 +5078,13 @@ unsigned long i915_gem_obj_offset(struct drm_i915_gem_object *o,
 	    vm == &dev_priv->mm.aliasing_ppgtt->base)
 		vm = &dev_priv->gtt.base;
 
-	BUG_ON(list_empty(&o->vma_list));
 	list_for_each_entry(vma, &o->vma_list, vma_link) {
 		if (vma->vm == vm)
 			return vma->node.start;
 
 	}
+	WARN(1, "%s vma for this object not found.\n",
+	     i915_is_ggtt(vm) ? "global" : "ppgtt");
 	return -1;
 }