drm/radeon: move bl encoder assignment into bl init

So that the bl encoder will be null if the GPU does not
control the backlight.

Reviewed-by: Michel Dänzer <michel.daenzer@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/radeon/atombios_encoders.c

@@ -237,6 +237,7 @@ void radeon_atom_backlight_init(struct radeon_encoder *radeon_encoder,
 	backlight_update_status(bd);
 
 	DRM_INFO("radeon atom DIG backlight initialized\n");
+	rdev->mode_info.bl_encoder = radeon_encoder;
 
 	return;