drm/i915/gvt: Assign NULL to the pointer after memory free.

The klocwork static code analyzer complains about using pointer after
being freed, because further we pass it to the gvt_vgpu_err() function.
Assign pointer to be NULL intentionaly, to meet requirements of the code
analyzer.

This patch fixed the issue #648 reported as error by klocwork.

Acked-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Aleksei Gimbitskii <aleksei.gimbitskii@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhi Wang <zhi.a.wang@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>