drm/amdxcp: fix amdxcp unloads incompletely

amdxcp unloads incompletely, and below error will be seen during load/unload,
sysfs: cannot create duplicate filename '/devices/platform/amdgpu_xcp.0'

devres_release_group will free xcp device at first, platform device will be
unregistered later in platform_device_unregister.

Signed-off-by: James Zhu <James.Zhu@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>