drm: Introduce drm_connector_register_all() helper

As a pair to already existing drm_connector_unregister_all() we're adding
generic implementation of what is already done in some drivers.

Once this helper is implemented we'll be ready to switch existing
driver-specific implementations with the generic one.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1461068693-11260-2-git-send-email-abrodkin@synopsys.com