drm/i915/selftests: fix smatch warning in mock_reserve

If mock_region_create fails then mem will be an error pointer. Instead
we just need to use the correct ordering for the onion unwind.

igt_mock_reserve() error: 'mem' dereferencing possible ERR_PTR()

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Ramalingam C <ramalingam.c@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210702104642.1189978-2-matthew.auld@intel.com