drm/msm/dsi-staging: improve xlog debug support for dsi

This change adds more debug points in xlog for dsi
driver and also adds support to dump only dsi specific
registers along with dsi debug bus registers instead of
relying on full register dump every time.

Change-Id: I27c122e7aea146af8a0241142486a7cf5a408e80
Signed-off-by: Sandeep Panda <spanda@codeaurora.org>