drm/i915/gvt: remove flag F_CMD_ACCESSED

Flag F_CMD_ACCESSED is not used. just remove it.

Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20200811063744.3272-1-yan.y.zhao@intel.com