ipa: rkisp1: agc: Move AeEnable control to the AGC algorithm

The AGC algorithm implements the AeEnable control at runtime. Move the
declaration of the control from the IPA module to the algorithm.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>