RC6p on Sandy Bridge got re-enabled over time, causing visual glitches
and GPU hangs.

Disabled originally in commit 1c8ecf80 ("drm/i915: do not enable
RC6p on Sandy Bridge").
Signed-off-by: Sasa Dragic <sasa.dragic@gmail.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221219172927.9603-2-sasa.dragic@gmail.com
Fixes: fb6db0f5 ("drm/i915: Remove unsafe i915.enable_rc6")
Fixes: 13c5a577 ("drm/i915/gt: Select the deepest available parking mode for rc6")
Cc: stable@vger.kernel.org
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>