drm: Fix regression in disable polling e58f637b

I broke out my trusty i845 and found a new boot failure, which upon
inspection turned out to be a recursion within:

drm_helper_probe_single_connector_modes() -> drm_helper_hpd_irq_event()
-> intel_crt_detect() -> drm_helper_probe_single_connector_modes()

Calling drm_kms_helper_poll_enable() instead performs the desired
re-initialisation of the polling should the user have toggled the
parameter, without the recursive side-effect.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>