drm/fb_helper: Use add_one_connector in add_all_connectors.

add_all_connectors doesn't checks whether reallocation is needed, but add_one does.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1455540316-17066-2-git-send-email-maarten.lankhorst@linux.intel.com