drm/i915: use for_each_intel_connector_iter in intel_display.c

This gets rid of the last users of for_each_intel_connector(), remove
that too.

At first I wasn't sure whether the 2 loops in the modeset state
checker should instead only loop over the connectors in the atomic
commit. But we never add connectors to an atomic update if they don't
(or won't have) a CRTC assigned, which means there'd be a gap in check
coverage. Hence loop over everything on those too.

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>
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/20170301095226.30584-5-daniel.vetter@ffwll.ch