When an output is enabled and modeset at the same time,
drm_connector_commit would first try to modeset then try to commit. This
won't work because both will trigger a page-flip. KMS will reject that.
Change the logic to only enable an output if no modeset has been
requested. The logic in wlr_output already checks that the user isn't
doing a modeset and disabling the output at the same time.