drm/i915/gvt: Fix cached atomics setting for Windows VM

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