drm/i915: dp: fix order of dp aux i2c device cleanup

drm/i915: dp: fix order of dp aux i2c device cleanup

Atm we set the parent of the dp i2c device to be the correspondig
connector device. During driver cleanup we first remove the connector
device through intel_modeset_cleanup()->drm_sysfs_connector_remove() and
only after that the i2c device through the encoder's destroy callback.
This order is not supported by the device core and we'll get a warning,
see the below bugzilla ticket. The proper order is to remove first any
child device and only then the parent device.

The first part of the fix changes the i2c device's parent to be the drm
device. Its logical owner is not the connector anyway, but the encoder.
Since the encoder doesn't have a device object, the next best choice is
the drm device. This is the same what we do in the case of the sdvo i2c
device and what the nouveau driver does.

The second part creates a symlink in the connector's sysfs directory
pointing to the i2c device. This is so, that we keep the current ABI,
which also makes sense in case someone wants to look up the i2c device
belonging to a specific connector.

Reference: http://lists.freedesktop.org/archives/intel-gfx/2014-January/038782.html
Reference: http://lists.freedesktop.org/archives/intel-gfx/2014-February/039427.html
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=70523
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Antti Koskipää <antti.koskipaa@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>