drm/i915: Use intel_panel for DVO fixed mode handling

Replace intel_dvo->panel_fixed_mode with the appropriate intel_panel
stuff. Now all connectors that have a fixed mode use intel_panel.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Mika Kahola <mika.kahola@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>