drm/i915/sdvo: prefer __packed over __attribute__((packed))

The kernel preference is to use the __packed macro instead of the direct
__attribute__.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220317173355.336835-1-jani.nikula@intel.com