drm/amd/display: Pass proper parent for DM backlight device registration

The parent for the backlight device should be the drm-connector object,
not the PCI device.

Userspace relies on this to be able to detect which backlight class device
to use on hybrid gfx devices where there may be multiple native (raw)
backlight devices registered.

Specifically gnome-settings-daemon expects the parent device to have
an "enabled" sysfs attribute (as drm_connector devices do) and tests
that this returns "enabled" when read.

This aligns the parent of the backlight device with i915, nouveau, radeon.
Note that drivers/gpu/drm/amd/amdgpu/atombios_encoders.c also already
uses the drm_connector as parent, only amdgpu_dm.c used the PCI device
as parent before this change.

Note this is marked as a RFC because I don't have hw to test, so this
has only been compile tested! If someone can test this on actual
hw which hits the changed code path that would be great.

Link: https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/issues/730


Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>