drm/amd/display/amdgpu_dm: Move most backlight setup into setup_backlight_device()

Rename register_backlight_device() to setup_backlight_device()
and move all backlight setup related calls from
amdgpu_dm_register_backlight_device() and from
amdgpu_dm_initialize_drm_device() there.

This leaves amdgpu_dm_register_backlight_device() dealing purely
with registering the actual backlight class device.

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>