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@embeddedgus


Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>