msm: mdss: add debug bus support for dsi block

Add support to dump dsi debug bus registers to memory
or print in xlog. This will be helpful in analyzing HW
behavior more precisely while debugging DSI stability
issues.

Change-Id: I7735897d9f24ca06e15015cf0d277bfe800b35bf
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>