drm/i915/selftests: Fix an IS_ERR() vs NULL check

The live_context() function returns error pointers.  It never returns
NULL.

Fixes: 9c1477e8

 ("drm/i915/selftests: Exercise adding requests to a full GGTT")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190326050843.GA20038@kadam
(cherry picked from commit 602cbe8e

)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>