drm/i915: checking for NULL instead of IS_ERR() in mock selftests

i915_gem_request_alloc() uses error pointers.  It never returns NULLs.

Fixes: 0daf0113 ("drm/i915: Mock infrastructure for request emission")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170413195217.GA26108@mwanda


Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit be02f755)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>