msm: mdss: add real time debug log

To analyze issues like underruns and other timing issues
add xlogs. Add debugfs entry for xlog to enable or disable
logging and register dumping at run time.

Enable xlog: echo 1 > <debugfs>/mdp/xlog/enable

Enable register dump:
echo 1 > <debugfs>/mdp/xlog/reg_dump

Enable panic on error:
echo 1 > <debugfs>/mdp/xlog/panic

Get xlogs and register dump
cat <debugfs>/mdp/xlog/dump

Change-Id: I10c2993d9dde6ef16a37256928fff1239e6370ff
Signed-off-by: Raghavendra Ambadas <rambad@codeaurora.org>