drm/i915/bxt: DSI prepare changes for BXT

This patch modifies dsi_prepare() function to support the same
modeset prepare sequence for BXT also. Main changes are:
1. BXT port control register is different than VLV.
2. BXT modeset sequence needs vdisplay and hdisplay programmed
   for transcoder.
3. BXT can select PIPE for MIPI transcoders.
4. BXT needs to program register MIPI_INIT_COUNT for both the ports,
   even if only one is being used.

v2: Fixed Jani's review comments. Rectified the DSI Macros to get
    proper register offsets using _MIPI_PORT instead of _TRANSCODER

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>