drm/i915/gt: Use the correct err_unlock unwind path for a closed context

A silly cut'n'paste copied the unlocked error path and used it inside
the pin_mutex lock, we need to drop that lock before returning.

Fixes: b412c63f

 ("drm/i915/gt: Report context-is-closed prior to pinning")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200322123241.17694-1-chris@chris-wilson.co.uk