drm/i915/selftests: Fix an error handling path in 'mock_gem_device()'

drm/i915/selftests: Fix an error handling path in 'mock_gem_device()'

Goto the right label in case of error, otherwise there is a leak.
This has been introduced by d44ba950ffb3. In this patch a goto has not been
updated.

Fixes: d44ba950ffb3 ("drm/i915: Create a kmem_cache to allocate struct i915_priolist from")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20170719223503.30580-1-christophe.jaillet@wanadoo.fr
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit a5ec7fe81a6ec38cb8b8a798d0552cbcadce7aa9)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>