dlist: fix pop_head to work for any size dlist

Created by: Elieva

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