drm/i915/gvt: fix mem leakage in setup_vgpu_mmio for vgpu reset

Gvt gdrst handler handle_device_reset() invoke function
setup_vgpu_mmio() to reset mmio status. In this case,
the virtual mmio memory has been allocated already. The
new allocation just cause old mmio memory leakage.

Signed-off-by: Du, Changbin <changbin.du@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>