drm/i915: Use drm_connector_list_iter in debugfs

drm/i915: Use drm_connector_list_iter in debugfs

While at it also try to reduce the locking a bit to what's really just
needed instead of everything that we could possibly lock.

Added a new for_each_intel_connector_iter which includes the cast to
intel_connector.

Otherwise just plain transformation with nothing special going on.

v2: Review from Maarten:
- Stick with modeset_lock_all in sink_crc, it looks at crtc->state.
- Fix up early loop exit in i915_displayport_test_active_write.

v3: 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> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170301095226.30584-1-daniel.vetter@ffwll.ch