drm: remove unsafe drm_for_each_connector()

After converting all users to drm_for_each_connector_iter() we no
longer need drm_for_each_connector() so we can go ahead and remove it.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170511191049.28944-8-gustavo@padovan.org