This patch adds helper disengcfg_sig_select() support so that users may select different taps(FrameGen, GammaCor, Matrix or Dither) to do signature computation. Also, select FrameGen as the default tap in _dpu_dec_init() and call it in dpu_dec_init(). Signed-off-by: Liu Ying <victor.liu@nxp.com> Reviewed-by: Robby Cai <robby.cai@nxp.com>