msm: mdss: debugfs: xlog: add iommu xlog

Add iommu xlog flags and handlers. MDSS_XLOG_IOMMU is dedicated to
iommu issues. The fault handler, if installed, dumps mdp registers to
ram, dump xlog entries to kernel log and panic the system.

Change-Id: I7d703c5958104fb3dc2c30a7fc1e59508c8d0ca3
Signed-off-by: Huaibin Yang <huaibiny@codeaurora.org>
Signed-off-by: Ingrid Gallardo <ingridg@codeaurora.org>