drm/xe: Introduce xe_gt_is_main_type helper

Instead of checking for not being a media type GT provide a small
helper to explicitly express our intentions.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Piotr Piórkowski <piotr.piorkowski@intel.com>
Link: https://lore.kernel.org/r/20250713103625.1964-5-michal.wajdeczko@intel.com