drm/virtio: make drm_fb_helper_funcs const

Make these structures const as they are only passed to the function
drm_fb_helper_prepare and the corresponding argument is of type const.
Done using Coccinelle

@match disable optional_qualifier@
identifier s;
@@
static struct drm_fb_helper_funcs s = {...};

@ref@
position p;
identifier match.s;
@@
s@p

@good1@
identifier match.s;
expression e1,e2;
position ref.p;
@@
drm_fb_helper_prepare(e1,e2,&s@p,...)

@bad depends on  !good1@
position ref.p;
identifier match.s;
@@
s@p

@depends on forall !bad disable optional_qualifier@
identifier match.s;
@@
static
+ const
struct drm_fb_helper_funcs s;
Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1502205245-17365-1-git-send-email-bhumirks@gmail.com

drivers/gpu/drm/virtio/virtgpu_fb.c

@@ -308,7 +308,7 @@ static int virtio_gpu_fbdev_destroy(struct drm_device *dev,
 
 	return 0;
 }
-static struct drm_fb_helper_funcs virtio_gpu_fb_helper_funcs = {
+static const struct drm_fb_helper_funcs virtio_gpu_fb_helper_funcs = {
 	.fb_probe = virtio_gpufb_create,
 };