dlist: fix pop_head to work for any size dlist

If a dlist contained more than two nodes, the previous link
of the new list head was not updated and pointed still to the
popped head.

Change-Id: I19507a6910d714f441290db8de5481113847370d
Signed-off-by: Elieva Pignat <Elieva.Pignat@arm.com>