msm: mdss: debugfs: xlog: dump mdp registers to RAM

Currently xlog provides the capability of dumping mdp registers to
kernel log. This patch adds the option of dumping mdp regs to ram as
well.

echo 0x0 > <debugfs>/mdp/xlog/reg_dump -> disable mdp reg dump
echo 0x1 > <debugfs>/mdp/xlog/reg_dump -> enable mdp reg dump to log
echo 0x2 > <debugfs>/mdp/xlog/reg_dump -> enable mdp reg dump to ram
echo 0x3 > <debugfs>/mdp/xlog/reg_dump -> enable mdp reg dump to both

Change-Id: I767bbd956ac3b1c9e76800bd9d563fc80bd337db
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>