drm/xe: hold mem_access.ref for CT fast-path

Just checking xe_device_mem_access_ongoing() is not enough, we also need
to hold the reference otherwise the ref can transition from 1 -> 0 as we
enter g2h_read(), leading to warnings. While we can't do a full rpm sync
in the IRQ, we can keep the device awake if the ref is non-zero.
Introduce a new helper for this and set it to work in for the CT
fast-path.

Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: José Roberto de Souza <jose.souza@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>