drm/xe: Use atomic instead of mutex for xe_device_mem_access_ongoing

xe_guc_ct_fast_path() is called from an irq context, and cannot lock
the mutex used by xe_device_mem_access_ongoing().

Fortunately it is easy to fix, and the atomic guarantees are good enough
to ensure xe->mem_access.hold_rpm is set before last ref is dropped.

As far as I can tell, the runtime ref in device access should be
killable, but don't dare to do it yet.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Acked-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>