drm/i915/gt: Enable read back on XEHP_FF_MODE2

Contrary to GEN12_FF_MODE2, platforms using XEHP_FF_MODE2 are not
affected by Wa_1608008084, hence read back can be enabled.

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20230630203509.1635216-6-lucas.demarchi@intel.com