Issue:
Will have follow error when reload driver:
[ 3986.567739] sysfs: cannot create duplicate filename '/devices/pci0000:00/0000:00:07.0/drm_dp_aux_dev'
[ 3986.567743] CPU: 6 PID: 1767 Comm: modprobe Tainted: G           OE     5.0.0-rc1-custom #1
[ 3986.567745] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
[ 3986.567746] Call Trace:
......
[ 3986.567808]  drm_dp_aux_register_devnode+0xdc/0x140 [drm_kms_helper]
......
[ 3986.569081] kobject_add_internal failed for drm_dp_aux_dev with -EEXIST, don't try to register things with the same name in the same directory.

Reproduce sequences:
1.modprobe amdgpu
2.modprobe -r amdgpu
3.modprobe amdgpu

Root cause:
When unload driver, it doesn't unregister aux.

v2: Don't use has_aux
Signed-off-by: Emily Deng <Emily.Deng@amd.com>
Reviewed-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>