drm/i915/gvt: remove unused vblank_done completion

This variable is used nowhere, so remove it.

Cc: Zhenyu Wang <zhenyuw@linux.intel.com>

Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Julian Stecklina <julian.stecklina@cyberus-technology.de>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200109171357.115936-3-julian.stecklina@cyberus-technology.de