If we have no fence and desire no fence on the vma, return before we try
and take the vm->mutex.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200108153550.3803446-1-chris@chris-wilson.co.uk