drm/gma500: Use backlight_get_brightness() to get the brightness

Use backlight_get_brightness() instead of directly referencing
bd->props.brightness. This will take backlight_is_blank() into account,
properly setting brightness to 0 when screen-blanking has been requested
through the backlight sysfs interface.

Suggested-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220917205920.647212-4-hdegoede@redhat.com