drm/xe/hw_engine_group: Don't use drm_warn to catch missed case

Since hwe->class is an enumeration we can rely on the compiler
to catch any unhandled engine class case at compile time thanks
to [-Werror=switch]. Any unexpected use of a special CLASS_MAX
enum case can be guarded by our xe_gt_assert() instead, which
will be compiled-out on the production builds.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Francois Dugast <francois.dugast@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Francois Dugast <francois.dugast@intel.com>
Link: https://lore.kernel.org/r/20250725090508.571-1-michal.wajdeczko@intel.com