drm/xe/guc: Copy response data from proper registers

While copying GuC response from the scratch registers to the buffer,
formula to identify next scratch register is broken. Fix it.

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>