drm/i915: use drm_connector_list_iter in intel_opregion.c

One case where I nuked a now unecessary locking, otherwise all just
boring stuff.

v2: Rebase onto the iter_get/put->iter_begin/end rename.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170301095226.30584-3-daniel.vetter@ffwll.ch