platform: remove unused REG_DSI macro

REG_DSI macro is no longer supported for MDSS-based targets since
the preferred approach for DSI register programming is by specifying
and offset with respect to a particular DSI controller's base. Remove
these macros from corresponding platforms.

Change-Id: I39eda8e435f55548d34fc64b5687a1ca71bd5e56
6 files changed