drm/i915/gvt: Adding ACTHD mmio read handler

When a workload is too heavy to finish it in gpu hang check timer
intervals(1.5), gpu hang check function will check ACTHD register
value to decide whether gpu is real dead or not. On real hw,
ACTHD is updated by HW when workload is running, then host kernel
won't think it is gpu hang. while guest kernel always read a constant
ACTHD value as GVT doesn't supply ACTHD emulate handler, then
guest kernel detects a fake gpu hang.

To remove such guest fake gpu hang, this patch supply ACTHD
mmio read handler which read real HW ACTHD register directly.

Signed-off-by: Xiong Zhang <xiong.y.zhang@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b4c9a097-3e62-124e-6856-b0c37764df7b@intel.com