drm/i915: Reduce WARN(i915_gem_valid_gtt_space) to a debug-only check

i915_gem_valid_gtt_space() is used after inserting the VMA to double
check the list - the location should have been chosen to pass all the
restrictions.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-8-git-send-email-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/i915_gem.c

@@ -3090,10 +3090,7 @@ search_free:
 			goto err_vma;
 		}
 	}
-	if (WARN_ON(!i915_gem_valid_gtt_space(vma, obj->cache_level))) {
-		ret = -EINVAL;
-		goto err_remove_node;
-	}
+	GEM_BUG_ON(!i915_gem_valid_gtt_space(vma, obj->cache_level));
 
 	trace_i915_vma_bind(vma, flags);
 	ret = i915_vma_bind(vma, obj->cache_level, flags);