drm/i915: re-disable RC6p on Sandy Bridge

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>

drivers/gpu/drm/i915/i915_pci.c

@@ -423,7 +423,8 @@ static const struct intel_device_info ilk_m_info = {
 	.has_coherent_ggtt = true, \
 	.has_llc = 1, \
 	.has_rc6 = 1, \
-	.has_rc6p = 1, \
+	/* snb does support rc6p, but enabling it causes various issues */ \
+	.has_rc6p = 0, \
 	.has_rps = true, \
 	.dma_mask_size = 40, \
 	.__runtime.ppgtt_type = INTEL_PPGTT_ALIASING, \