drm/xe/guc: Improve GuC doorbell/context ID manager intro message

We can use recently added str_plural() helper.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419153407.402-1-michal.wajdeczko@intel.com