drm/i915/dg2: Add DG2 to the PSR2 defeature list

PSR2 is not supported on DG2.

Cc: Caz Yokoyama <Caz.Yokoyama@intel.com>
Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Anusha Srivatsa <anusha.srivatsa@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210714031540.3539704-49-matthew.d.roper@intel.com