drm/i915/gtt: Reference mappable_end variable from pointer

Reference variable value from pointer, not assumed pointer destination.

Since:

commit c44ef60e437019b8ca1dab8b4d2e8761fd4ce1e9
Author: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Date:   Thu Jun 25 18:35:05 2015 +0300

    drm/i915/gtt: Allow >= 4GB sizes for vm.

Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -3103,8 +3103,7 @@ static int gen6_gmch_probe(struct drm_device *dev,
 	 * a coarse sanity check.
 	 */
 	if ((*mappable_end < (64<<20) || (*mappable_end > (512<<20)))) {
-		DRM_ERROR("Unknown GMADR size (%llx)\n",
-			  dev_priv->gtt.mappable_end);
+		DRM_ERROR("Unknown GMADR size (%llx)\n", *mappable_end);
 		return -ENXIO;
 	}