drm/i915/gvt: clear ggtt entries when destroy vgpu

When one vgpu is destroyed, its ggtt entries are not cleared.
This patch clears ggtt entries to avoid information leak.

v2: add 'Fixes' tag (Zhenyu)

Fixes: 2707e444

 ("drm/i915/gvt: vGPU graphics memory virtualization")
Signed-off-by: Zhipeng Gong <zhipeng.gong@intel.com>
Reviewed-by: Hang Yuan <hang.yuan@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>