drm/amd/display/amdgpu_dm: Make amdgpu_dm_register_backlight_device() take an amdgpu_dm_connector

Make amdgpu_dm_register_backlight_device() take an amdgpu_dm_connector
pointer to the connector for which it should register the backlight
as its only argument.

This is a preparation patch for moving the actual backlight class device
registering to drm_connector_funcs.late_register.

Reviewed-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>