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

drivers/gpu/drm/rockchip/analogix_dp-rockchip.c

@@ -262,7 +262,7 @@ rockchip_dp_drm_encoder_atomic_check(struct drm_encoder *encoder,
 	return 0;
 }
 
-static struct drm_encoder_helper_funcs rockchip_dp_encoder_helper_funcs = {
+static const struct drm_encoder_helper_funcs rockchip_dp_encoder_helper_funcs = {
 	.mode_fixup = rockchip_dp_drm_encoder_mode_fixup,
 	.mode_set = rockchip_dp_drm_encoder_mode_set,
 	.atomic_enable = rockchip_dp_drm_encoder_enable,

drivers/gpu/drm/rockchip/inno_hdmi.c

@@ -545,7 +545,7 @@ inno_hdmi_encoder_atomic_check(struct drm_encoder *encoder,
 	return 0;
 }
 
-static struct drm_encoder_helper_funcs inno_hdmi_encoder_helper_funcs = {
+static const struct drm_encoder_helper_funcs inno_hdmi_encoder_helper_funcs = {
 	.atomic_check	= inno_hdmi_encoder_atomic_check,
 	.atomic_enable	= inno_hdmi_encoder_enable,
 	.atomic_disable	= inno_hdmi_encoder_disable,