msm: mdss: debug: fix xlog crash during dump

Fix to initialize the nodes added by the panel
data dump to the xlog linked list.
This resolves a crash when runtime dumping
the registers through xlog with the dump
debugfs node:
echo 1 > /d/mdp/xlog/dump

Change-Id: I2a7222bd10e5d5a53b20472fbfbc3db818897515
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>