drm/i915/gvt: print correct value for untracked mmio

In function intel_vgpu_emulate_mmio_read, the untracked mmio register is
dumped through kernel log, but the register value is not correct. This
patch fixes this issue.

V2: fix the fromat warning from checkpatch.pl.

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