drm/i915: Sanitize i915_get_bridge_dev() error path

Clarify the name of the label on the error path, making it clear what's
being cleaned up. The kmem_cache_destroy() calls are NOPs on the
corresponding error path.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: David Weinehall <david.weinehall@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453209992-25995-3-git-send-email-imre.deak@intel.com