drm/xen-front: Make structure fb_funcs constant

Static structure fb_funcs, of type drm_framebuffer_funcs, is used only
when it is passed to drm_gem_fb_create_with_funcs() as its last
argument. drm_gem_fb_create_with_funcs does not modify its lst argument
(fb_funcs) and hence fb_funcs is never modified. Therefore make fb_funcs
constant to protect it from further modification.
Issue found with Coccinelle.

Signed-off-by: Nishka Dasgupta <nishkadg.linux@gmail.com>
Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190813062712.24993-1-nishkadg.linux@gmail.com

drivers/gpu/drm/xen/xen_drm_front_kms.c

@@ -46,7 +46,7 @@ static void fb_destroy(struct drm_framebuffer *fb)
 	drm_gem_fb_destroy(fb);
 }
 
-static struct drm_framebuffer_funcs fb_funcs = {
+static const struct drm_framebuffer_funcs fb_funcs = {
 	.destroy = fb_destroy,
 };