drm/xe/guc: Use xe_device_is_l2_flush_optimized()

drm/xe/guc: Use xe_device_is_l2_flush_optimized()

We encapsulate the logic to check if the platform has L2 flush
optimization feature in xe_device_is_l2_flush_optimized(), but
guc_ctl_feature_flags() is using an open-coded version of that same type
of check.  Fix that by replacing the open-coded check with
xe_device_is_l2_flush_optimized().

Reviewed-by: Himanshu Girotra <himanshu.girotra@intel.com>
Reviewed-by: Tejas Upadhyay <tejas.upadhyay@intel.com>
Link: https://patch.msgid.link/20260513-guc-l2-flush-opt-use-xe_device_is_l2_flush_optimized-v1-1-36fa866d6ed8@intel.com
Signed-off-by: Gustavo Sousa <gustavo.sousa@intel.com>