drm/rockchip: Constify struct drm_encoder_helper_funcs

'struct drm_encoder_helper_funcs' is not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   7458	    552	      0	   8010	   1f4a	drivers/gpu/drm/rockchip/analogix_dp-rockchip.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   7578	    424	      0	   8002	   1f42	drivers/gpu/drm/rockchip/analogix_dp-rockchip.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/128f9941aab3b1367eb7abca4ac26e2e5dd6ad21.1720903899.git.christophe.jaillet@wanadoo.fr