drm/i915/sdvo: Nuke attached_output tracking

Instead of operating on the output the user specified (via the
connector) the current code tends to operate on whichever outputs
it has detected as attached. That is not how the kms uapi is supposed
to work. So simply get rid of attached_outputs and instead directly
operate on the output the user has specified.

Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230825134431.24391-2-ville.syrjala@linux.intel.com