drm/amd/display: Use connector kdev as aux device parent

Set the connector's kernel device as the parent for the aux kernel
device. This allows udev rules to access connector attributes when
creating symlinks to aux devices.

For example, the following udev rule:

SUBSYSTEM=="drm_dp_aux_dev", SUBSYSTEMS=="drm", ATTRS{edid}=="*",
	SYMLINK+="drm_dp_aux/by-name/$id"

Will create the following symlinks using the connector's name:

$ ls /dev/drm_dp_aux/by-name/
card0-DP-1  card0-DP-2  card0-DP-3

Cc: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Cc: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Cc: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723232808.28128-6-sunpeng.li@amd.com