drm: Constify struct drm_fb_helper_funcs

There's no need for this to be modifiable. Make it const so that it can
be put into the .rodata section.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/qxl/qxl_fb.c

@@ -660,7 +660,7 @@ static int qxl_fbdev_destroy(struct drm_device *dev, struct qxl_fbdev *qfbdev)
 	return 0;
 }
 
-static struct drm_fb_helper_funcs qxl_fb_helper_funcs = {
+static const struct drm_fb_helper_funcs qxl_fb_helper_funcs = {
 	.fb_probe = qxl_fb_find_or_create_single,
 };