This is the logical place to put the backlight device, and it also
fixes a kernel crash if the MIPI host is removed. Previously the
backlight device would be unregistered twice when this happened - once
as a child of the MIPI host through `mipi_dsi_host_unregister`, and
once when the panel device is destroyed.

Fixes: 12a6cbd4 ("drm/panel: otm8009a: Use new backlight API")
Signed-off-by: James Cowgill <james.cowgill@blaize.com>
Cc: stable@vger.kernel.org
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230412173450.199592-1-james.cowgill@blaize.com