drm/i915/gvt: handle fence reg access during GPU reset

Lots of reduntant log info will be printed out during GPU reset,
including accessing untracked mmio register and fence register,
variable disable_warn_untrack is added previously to handle the
situation, but the accessing of fence register is ignored in the
previously patch, so add it back.

Besides, set the variable disable_warn_untrack to the defalut value
after GPU reset is finished.

Signed-off-by: Zhao, Xinda <xinda.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>