drm/i915: add MIPI DSI register definitions

Add definitions for VLV MIPI DSI registers.

v2: Small fixes per Ville's review comments.
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>