atmel_lcdfb: blank the backlight on remove

When removing atmel_lcdfb module, the backlight is unregistered but not
blanked. (only for CONFIG_BACKLIGHT_ATMEL_LCDC case).
This can result in the screen going full white depending on how the PWM
is wired.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>