drm/xe: Fix a NULL vs IS_ERR() in xe_vm_add_compute_exec_queue()

The xe_preempt_fence_create() function returns error pointers.  It
never returns NULL.  Update the error checking to match.

Fixes: dd08ebf6 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Link: https://lore.kernel.org/r/aJTMBdX97cof_009@stanley.mountain


Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>