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.

Changes in v3:
Make amdgpu_dm_register_backlight_device() check bl_idx != 1 before
registering the backlight since amdgpu_dm_connector_late_register()
now calls it for _all_ connectors.

Changes in v2:
Together with changing the parent, also move the registration to
drm_connector_funcs.late_register() this is necessary because the parent
device (which now is the drm_connector) must be registered before
the backlight class device is, otherwise the backlight class device ends
up without any parent set at all.

This brings the backlight class device registration timing inline with
nouveau and i915 which also use drm_connector_funcs.late_register()
for this.

Note this slightly changes backlight_device_register() error handling,
instead of not increasing dm->num_of_edps and re-using the current
bl_idx for a potential other backlight device, dm->backlight_dev[bl_idx]
is now simply left NULL on failure. This is ok because all code
looking at dm->backlight_dev[i] also checks it is not NULL.

Link: https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/issues/730Reviewed-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>