drm/i915/gem: Fix error path to unlock if the GEM context is closed

When inside the lock, remember to unlock even if you want to leave
early.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: a4e7ccda ("drm/i915: Move context management under GEM")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191106144155.25727-1-chris@chris-wilson.co.uk