omapdrm: hdmi4: refcount hdmi_power_on/off_core

The hdmi_power_on/off_core functions can be called multiple times:
when the HPD changes and when the HDMI CEC support needs to power
the HDMI core.

So use a counter to know when to really power on or off the HDMI core.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>