drm/i915/selftests: fix inconsistent IS_ERR and PTR_ERR

Fix inconsistent IS_ERR and PTR_ERR in shrink_boom.
The proper pointer to use is _explode_ instead of _purge_.

This issue was detected with the help of Coccinelle.

Fixes: fe215c8b ("drm/i915/selftests: add missing gtt shrinker test")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180214211234.GA22341@embeddedgusReviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/selftests/i915_gem_gtt.c

@@ -927,7 +927,7 @@ static int shrink_boom(struct drm_i915_private *i915,
 
 		explode = fake_dma_object(i915, size);
 		if (IS_ERR(explode)) {
-			err = PTR_ERR(purge);
+			err = PTR_ERR(explode);
 			goto err_purge;
 		}