drm/i915/gvt: add 0x4dfc to gen9 save-restore list

0x4dfc is in-context mmio for gen9+, but each vm have different settings
need to add it to save-restore list along with other trtt registers

Fixes: 1786571 ("drm/i915/gvt: vGPU context switch")
Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>

drivers/gpu/drm/i915/gvt/mmio_context.c

@@ -114,6 +114,7 @@ static struct engine_mmio gen9_engine_mmio_list[] __cacheline_aligned = {
 	{RCS0, TRINVTILEDETCT, 0, true}, /* 0x4dec */
 	{RCS0, TRVADR, 0, true}, /* 0x4df0 */
 	{RCS0, TRTTE, 0, true}, /* 0x4df4 */
+	{RCS0, _MMIO(0x4dfc), 0, true},
 
 	{BCS0, RING_GFX_MODE(BLT_RING_BASE), 0xffff, false}, /* 0x2229c */
 	{BCS0, RING_MI_MODE(BLT_RING_BASE), 0xffff, false}, /* 0x2209c */