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

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: dd08ebf6c352 ("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>
(cherry picked from commit 75cc23ffe5b422bc3cbd5cf0956b8b86e4b0e162)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>