drm: armada: Constify drm_prop_enum_list structures.

drm_prop_enum_lists are not supposed to change at runtime. All functions
working with drm_prop_enum_list provided by <drm/drm_property.h> work
with
const drm_prop_enum_list. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   3594	    176	      0	   3770	    eba drivers/gpu/drm/armada/armada_overlay.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   3722	     48	      0	   3770	    eba drivers/gpu/drm/armada/armada_overlay.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/dbeb176765bda8eaa9efdaa2dcd14c7bbae39cfa.1498905467.git.arvind.yadav.cs@gmail.com

drivers/gpu/drm/armada/armada_overlay.c

@@ -388,7 +388,7 @@ static const uint32_t armada_ovl_formats[] = {
 	DRM_FORMAT_BGR565,
 };
 
-static struct drm_prop_enum_list armada_drm_colorkey_enum_list[] = {
+static const struct drm_prop_enum_list armada_drm_colorkey_enum_list[] = {
 	{ CKMODE_DISABLE, "disabled" },
 	{ CKMODE_Y,       "Y component" },
 	{ CKMODE_U,       "U component" },