drm/i915/dvo: rename dvo.h to intel_dvo_dev.h and make self-contained

Put the header more in line with the rest. No functional changes.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/9f4dc21928f9cccd7a3593a2f9faa44b4412ff33.1556540889.git.jani.nikula@intel.com