drm/i915/bxt: Initialize MIPI DSI for BXT

This patch contains following changes:
1. Add BXT MIPI display address base.
2. Call dsi_init from display_setup function.

v2: Rebased on latest nightly branch

v3 by Jani: init dsi after ddi

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1458640910-5338-1-git-send-email-jani.nikula@intel.com