drm/i915/gvt: add PLANE_KEYMAX regs to mmio track list

Running 4.15 Linux kernel in VM will cause host GVT reports
'untrack mmio 0x701a0' errror, which identifies the PLANE_KEYMAX
registers. Add them to track list.

v2: rebase to latest staging code.

Signed-off-by: Pei Zhang <pei.zhang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>