atmel_lcdfb: Use proper blanking on negative contrast polarity

If used with negative polarity the PWM unit cannot be disabled. This would
result in a full contrast screen.
Instead let the PWM unit enabled using 0x0 as compare value which darkens
the display.
In result no power saving is possible if inverted contrast polarity
is used.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>
Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>