drm: sti: sti_dvo: make of_device_ids const.

of_device_ids are not supposed to change at runtime. All functions
working with of_device_ids provided by <linux/of.h> work with const
of_device_ids. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   4222	    664	      0	   4886	   1316	drivers/gpu/drm/sti/sti_dvo.o

File size after constify dvo_of_match:
   text	   data	    bss	    dec	    hex	filename
   4638	    248	      0	   4886	   1316	drivers/gpu/drm/sti/sti_dvo.o
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Link: http://patchwork.freedesktop.org/patch/msgid/d5ac3cb4c43338419308d658b9bcb59b7540471e.1497868332.git.arvind.yadav.cs@gmail.com

drivers/gpu/drm/sti/sti_dvo.c

@@ -582,7 +582,7 @@ static int sti_dvo_remove(struct platform_device *pdev)
 	return 0;
 }
 
-static struct of_device_id dvo_of_match[] = {
+static const struct of_device_id dvo_of_match[] = {
 	{ .compatible = "st,stih407-dvo", },
 	{ /* end node */ }
 };