drm/xe: unlock on error path in xe_vm_add_compute_exec_queue()

Drop the "&vm->lock" before returning.

Fixes: 24f947d5 ("drm/xe: Use DRM GPUVM helpers for external- and evicted objects")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
(cherry picked from commit cf46019e)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>