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

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

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

Fixes: 0daf0113cff6 ("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 be02f7556447a0dee672acb5e462f03377b98ae8)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>