msm: mdss: debugfs: xlog: enable xlog with different flags

xlog buffer helps capture debug events, however, due to its small
size, one set of xlogs cannot solve all problems. It would be helpful
that when a specific issue happens, only one set of xlogs specific to
that particular issue is dumped and other irrelevant events can be
ignored. Accordingly different event handlers can be registered as
well for different issues.

echo 0x00 > <debugfs>/mdp/xlog/enable -> disable
echo 0x01 > <debugfs>/mdp/xlog/enable -> enable default xlogs
echo 0x80 > <debugfs>/mdp/xlog/enable -> enable all xlogs
echo 0xff > <debugfs>/mdp/xlog/enable -> enable xlogs regardless flags

Change-Id: Id1bf2a6f2009afdc63d74add652ce2722a3173a4
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>