drm: rcar-du: Use DRM-managed allocation for encoders

devm_kzalloc() is the wrong API to allocate encoders, as the lifetime of
the encoders is tied to the DRM device, not the device to driver
binding. drmm_kzalloc() isn't a good option either, as it would result
in the encoder being freed before being unregistered during the managed
cleanup of the DRM objects. Use a plain kzalloc(), and register a drmm
action to cleanup the encoder.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>