drm/i915: Remove dead code from gen8_pte_encode

Commit 9275277d

 ("drm/i915: use pat_index instead of cache_level")
added a dedicated gen12_pte_encode but forgot to remove the Gen12 specific
bit from gen8_pte_encode.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: 9275277d

 ("drm/i915: use pat_index instead of cache_level")
Cc: Fei Yang <fei.yang@intel.com>
Cc: Andi Shyti <andi.shyti@linux.intel.com>
Cc: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Fei Yang <fei.yang@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230707124644.3965281-1-tvrtko.ursulin@linux.intel.com