drm: Clean up connectors by unreferencing them

Only static connectors should be left at this point, and we should be
able to clean them out by simply dropping that last reference still
around from drm_connector_init.

If that leaves anything behind then we have a driver bug.

Doing the final cleanup this way also allows us to use
drm_connector_iter, removing the very last place where we walk
connector_list explicitly in drm core&helpers.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161213230814.19598-8-daniel.vetter@ffwll.ch