We've seen recent regression with host and windows VM running
simultaneously that cause gpu hang or even crash. Finally bisect to
commit 58586680 ("drm/i915: Disable atomics in L3 for gen9"),
which seems cached atomics behavior difference caused regression
issue.

This tries to add new scratch register handler and add those in mmio
save/restore list for context switch. No gpu hang produced with this one.

Cc: stable@vger.kernel.org # 5.12+
Cc: "Xu, Terrence" <terrence.xu@intel.com>
Cc: "Bloomfield, Jon" <jon.bloomfield@intel.com>
Cc: "Ekstrand, Jason" <jason.ekstrand@intel.com>
Reviewed-by: Colin Xu <colin.xu@intel.com>
Fixes: 58586680 ("drm/i915: Disable atomics in L3 for gen9")
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210806044056.648016-1-zhenyuw@linux.intel.com