drm/i915: Shrink sdvo_cmd_names

Pack the struct _sdvo_cmd_name to save 736 bytes of .rodata.

This is fine since the name pointers are used only for debug.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>