drm/xe: Fix possible UAF in guc_exec_queue_process_msg

drm/xe: Fix possible UAF in guc_exec_queue_process_msg

Store xe_device ahead of processing message as message can be free'd in
some cases.

v2:
 - Including missing local changes
v3:
 - Resend for CI

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/r/202407231445.rpisd1vA-lkp@intel.com/
Fixes: d930c19fdff3 ("drm/xe: Build PM into GuC CT layer")
Signed-off-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Himal Prasad Ghimiray <himal.prasad.ghimiray@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240724164341.1848954-1-matthew.brost@intel.com