drm/i915: WARN if the bios reserved range is bigger than stolen size

v2: Bail out if we hit the WARN_ON to avoid fallout later on. Spotted
by Chris Wilson.
Suggested-by: NChris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_stolen.c

@@ -215,6 +215,9 @@ int i915_gem_init_stolen(struct drm_device *dev)
 	if (IS_VALLEYVIEW(dev))
 		bios_reserved = 1024*1024; /* top 1M on VLV/BYT */
 
+	if (WARN_ON(bios_reserved > dev_priv->gtt.stolen_size))
+		return 0;
+
 	/* Basic memrange allocator for stolen space */
 	drm_mm_init(&dev_priv->mm.stolen, 0, dev_priv->gtt.stolen_size -
 		    bios_reserved);