msm: mdss: debugfs: xlog: move reg dump function to xlog

reg dump func is specifically used by xlog, so it should be static in
xlog file.

Change-Id: I14570e174b215d7ea5f77f0b333f65956b549222
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>