drm: Minimally initialise drm_dp_aux

When trying to split up the initialisation phase and the registration
phase, one immediate problem encountered is trying to use our own i2c
devices before registration with userspace (to read EDID during device
discovery). drm_dp_aux in particular only offers an interface for setting
up the device *after* we have exposed the connector via sysfs. In order
to break the chicken-and-egg problem, export drm_dp_aux_init() to
minimally prepare the i2c device for internal use before
drm_connector_register().

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Dave Airlie <airlied@redhat.com>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: dri-devel@lists.freedesktop.org
[danvet: Amend kerneldoc slightly.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1466152398-20157-3-git-send-email-chris@chris-wilson.co.uk