drm/panel: visionox-rm69299: Add backlight support

drm/panel: visionox-rm69299: Add backlight support

The shift6mq's variant supports controlling the backlight via DSI
commands. Use that if a max_brightness is set in the device specific
data.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@oss.qualcomm.com>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250910-shift6mq-panel-v3-3-a7729911afb9@sigxcpu.org