drm/xe: Add device flag to indicate standalone MERT

drm/xe: Add device flag to indicate standalone MERT

The MERT subsystem manages memory accesses between host and device. On
the Crescent Island platform, it requires direct management by the
driver.

Introduce a device flag and corresponding helpers to identify platforms
with standalone MERT, enabling proper initialization and handling.

Signed-off-by: Lukasz Laguna <lukasz.laguna@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Link: https://patch.msgid.link/20251124190237.20503-2-lukasz.laguna@intel.com