drm/i915: Skip stolen region initialisation if none is reserved

Paulo reported that if he set the amount of reserved memory to 0, then
we emitted a warning about a conflict before disabling our use of stolen
memory. This was introduced with

commit eaba1b8f
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Thu Jul 4 12:28:35 2013 +0100

    drm/i915: Verify that our stolen memory doesn't conflict

and is simply fixed by checking for a no reservation first.
Reported-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_stolen.c

@@ -201,6 +201,9 @@ int i915_gem_init_stolen(struct drm_device *dev)
 	struct drm_i915_private *dev_priv = dev->dev_private;
 	int bios_reserved = 0;
 
+	if (dev_priv->gtt.stolen_size == 0)
+		return 0;
+
 	dev_priv->mm.stolen_base = i915_stolen_to_physical(dev);
 	if (dev_priv->mm.stolen_base == 0)
 		return 0;