gsd: Control panel and function key should not set brightness to 0

The user could slide the brightness on the control panel or press the function key to make the screen brightness to 0.

However, If the user does not know about the keyboard buttons, he/she will have a hard time turning the display back from the control panel if the display is too dim or completely off. For example, systems like ASUS UX550GE's brightness is in the range from 1 to 120000. The brightness level 1 has the backlight level which is so dim and it's also practically unusable.

This patch makes it limiting to 1% brightness as the lower bound avoids that usability issue. #54 (closed)

Signed-off-by: Jian-Hong Pan jian-hong@endlessm.com