drm/i915/guc: The Additional Data Struct (ADS) should get enabled together with GuC submission

It's mandatory and it gets created if and only if GuC submission is enabled, so that should be
the condition for informing the GuC about it.

Also s/guc_addon_create/guc_ads_create and s/guc_addon_destroy/guc_ads_destroy and, while
at it, add an explanation of what things go inside the ADS object.

Signed-off-by: Oscar Mateo <oscar.mateo@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>