drm/i915: Convert fixed_mode/downclock_mode into a list

Store the fixed_mode and downclock_mode as a real list,
in preparation for exposing other supported modes as well.

v2: Init the list in intel_sdvo_connector_alloc() too
v3: Use list_first_entry_or_null() (Jani)

Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220314152737.9125-1-ville.syrjala@linux.intel.com