drm/omap: fix OMAP4 hdmi_core_powerdown_disable()

hdmi_core_powerdown_disable() is supposed to disable HDMI core's
power-down mode. However, the function sets the power-down bit to 0,
which means "enable power-down".

This hasn't caused any issues as the PD seems to affect only interrupts
from HDMI core, and none of those interrupts are used at the moment. CEC
functionality requires core interrupts, and the PD mode needs to be
fixed.

This patch fixes hdmi_core_powerdown_disable() to actually disable the
PD mode.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Hans Verkuil <hverkuil@xs4all.nl>