drm/i915: Change Mipi register definitions

Re-define MIPI register definitions in such a way that most of
the existing DSI code can be re-used for future platforms. Register
definitions are re-written using MMIO offset variable, so that without
changing the existing sequence, same code can be generically applied.

V4: Addressing review comments by Damien and Ville, splitting into two patches
This patch removes all the un-necessary formatting changes from previous patch.
V5: Removed 80 char limit formatting for existing MIPI regs
V6: Removed extra space, change one definition

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>