thunderbolt: Fix a backport error for display flickering issue

A mistake was made when backporting commit 583893a6 ("thunderbolt: Fix
Thunderbolt 3 display flickering issue on 2nd hot plug onwards") in missing
the `if` block.  Add it back in.

Reported-by:  <Joakim.Tjernlund@infinera.com>
Closes: https://lore.kernel.org/stable/28b5d0accce90bedf2f75d65290c5a1302225f0f.camel@infinera.com/


Fixes: 06614ca4 ("thunderbolt: Fix Thunderbolt 3 display flickering issue on 2nd hot plug onwards")
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>