backlight: fix a bug where the screen would be completely dark or near darkness with backlight_dimmer enabled when the display dimmed. The brightness level can't be any lower than dev->min_brightness and previous code allowed that to happen, so fix it.

Signed-off-by: franciscofranco <franciscofranco.1990@gmail.com>
Signed-off-by: flar2 <asegaert@gmail.com>