To support dynamic connector selection function, each ddp_comp need to
get their encoder_index to identify which connector should be selected.
Add encoder_index interface for mtk_ddp_comp_funcs to get the encoder
identifier by drm_encoder_index().
Then drm driver will call mtk_ddp_comp_encoder_index_set() to store the
encoder_index to each ddp_comp in connector routes.
Signed-off-by: Jason-JH.Lin <jason-jh.lin@xxxxxxxxxxxx>