video: fbdev: imxfb: fix semantic of .get_power and .set_power

.set_power gets passed an FB_BLANK_XXX value, not a bool. So 0 signals
on; and >1 means off. The same applies for return values of .get_power.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>