drm/i915/guc: Move notification code into virtual function

Prepare for alternate GuC notification mechanism.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
[Joonas: Added newlines]
Signed-off-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>