drm/i915/bxt: DSI enable for BXT

This patch contains following changes:
1. MIPI device ready changes to support dsi_pre_enable. Changes
   are specific to BXT device ready sequence. Added check for
   ULPS mode(No effects on VLV).
2. Changes in dsi_enable to pick BXT port control register.
3. Changes in dsi_pre_enable to restrict DPIO programming for VLV

v2: Fixed Jani's review comments. Removed the changes in VLV/CHV
    code. Fixed the macros to get proper port offsets.

v3: Rebased on latest drm-nightly branch. Fixed Jani's review comments.

Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
Signed-off-by: Uma Shankar <uma.shankar@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>