drm/probe-helper: Fix drm_kms_helper_poll_enable() docs

Fix docs to reflect code and drm_kms_helper_poll_disable() docs by saying
that calling drm_kms_helper_poll_enable() is fine even if output polling
is not enabled.

Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20171106191812.38927-2-noralf@tronnes.org