drm/exynos: Constify struct exynos_drm_ipp_funcs

'struct exynos_drm_ipp_funcs' are 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, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  20446	   1746	     16	  22208	   56c0	drivers/gpu/drm/exynos/exynos_drm_fimc.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  20446	   1714	     16	  22176	   56a0	drivers/gpu/drm/exynos/exynos_drm_fimc.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/exynos_drm_fimc.c

@@ -1125,7 +1125,7 @@ static void fimc_abort(struct exynos_drm_ipp *ipp,
 	}
 }
 
-static struct exynos_drm_ipp_funcs ipp_funcs = {
+static const struct exynos_drm_ipp_funcs ipp_funcs = {
 	.commit = fimc_commit,
 	.abort = fimc_abort,
 };

drivers/gpu/drm/exynos/exynos_drm_gsc.c

@@ -1162,7 +1162,7 @@ static void gsc_abort(struct exynos_drm_ipp *ipp,
 	}
 }
 
-static struct exynos_drm_ipp_funcs ipp_funcs = {
+static const struct exynos_drm_ipp_funcs ipp_funcs = {
 	.commit = gsc_commit,
 	.abort = gsc_abort,
 };

drivers/gpu/drm/exynos/exynos_drm_scaler.c

@@ -403,7 +403,7 @@ static int scaler_commit(struct exynos_drm_ipp *ipp,
 	return 0;
 }
 
-static struct exynos_drm_ipp_funcs ipp_funcs = {
+static const struct exynos_drm_ipp_funcs ipp_funcs = {
 	.commit = scaler_commit,
 };