msm: mdss: debugfs: xlog: move xlog related variables to local

debug_log is an unnecessary global structure that is actually only
related to xlog debugfs nodes. Those variables that control xlog
behaviors should be in the local xlog file.

Change-Id: I8e2a37e72d849b09e5c8afb8b2170adf9516e215
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>