drm: adv7511: Refactor power management

Remove the internal dependency on DPMS mode for power management by
using a by a powered state boolean instead, and use the new power off
handler at probe time. This ensure that the regmap cache is properly
marked as dirty when the device is probed, and the registers properly
synced during the first power up.

As a side effect this removes the initialization of current_edid_segment
at probe time, as the field will be initialized when the device is
powered on, at the latest right before reading EDID data.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Christian Kohn <christian.kohn@xilinx.com>
Tested-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>