drm/xe/guc: Flush G2H handler when turning off CTs

Make sure G2H handler is not running when changing the CT state to drop
messages or disabled. This will help prevent races in the code ensuring
that G2H are not being processed after changing the state.

v2:
- s/flush_g2h_handler/stop_g2h_handler (Michal)

Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
[Rodrigo remove the extra line while pushing]
Link: https://patchwork.freedesktop.org/patch/msgid/20240122210156.1517444-4-matthew.brost@intel.com