leds: aw2013_led: disable regulators when not needed

Turn off the LED regulators if brightness of all the LEDs that are
connected to the host is 0. This change is required to save the power
consumption.

Change-Id: I9bae343becb9a794a5ecc8ddf9c2edb3b5aecfce
Signed-off-by: Mao Li <maol@codeaurora.org>