drm/i915/gvt: fix memory leak of a cmd_entry struct on error exit path

The error exit path when a duplicate is found does not kfree and cmd_entry
struct and hence there is a small memory leak.  Fix this by kfree'ing it.

Detected by CoverityScan, CID#1370198 ("Resource Leak")

Fixes: be1da707 ("drm/i915/gvt: vGPU command scanner")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>