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>