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 c5cf9a9147ff. In this patch a goto has not been
updated.

Fixes: c5cf9a9147ff ("drm/i915: Create a kmem_cache to allocate struct i915_priolist from")
Signed-off-by: Christophe JAILLET <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
Reviewed-by: Tvrtko Ursulin <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>