drm/i915/dvo: Extract intel_dvo_regs.h

Pull the DVO port register definitons into their own header
to declutter i915_reg.h a bit.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221122120825.26338-10-ville.syrjala@linux.intel.com
Reviewed-by: Jani Nikula <jani.nikula@intel.com>