drm/i915/gvt: correct a false comment of flag F_UNALIGN

Correct falsely removed comment of flag F_UNALIGN.

Fixes: a6c5817a ("drm/i915/gvt: remove flag F_CMD_ACCESSED")
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/20200910035405.20273-1-yan.y.zhao@intel.com