drm/i915: constify intel_dvo_dev_ops structures

The intel_dvo_dev_ops structures are never modified, so declare them as
const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1449608127-21715-1-git-send-email-Julia.Lawall@lip6.fr