drm/i915/gvt: Add in context mmio 0x20D8 to gen9 mmio list

Depends on GEN family and I915_PARAM_HAS_CONTEXT_ISOLATION, Mesa driver
will decide whether constant buffer 0 address is relative or absolute,
and load GPU initial state by lri to context mmio INSTPM (GEN8)
or 0x20D8 (>=GEN9).
Mesa Commit fa8a764b62
("i965: Use absolute addressing for constant buffer 0 on Kernel 4.16+.")

INSTPM is already added to gen8_engine_mmio_list, but 0x20D8 is missed
in gen9_engine_mmio_list. From GVT point of view, different guest could
have different context so should switch those mmio accordingly.

v2: Update fixes commit ID.

Fixes: 17865713 ("drm/i915/gvt: vGPU context switch")
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>