drm/i915/bxt: Disable DSI PLL for BXT

This patch adds two new functions:
- disable_dsi_pll.
  BXT DSI disable sequence and registers are
  different from previous platforms.
- intel_disable_dsi_pll
  wrapper function to re-use the same code for
  multiple platforms. It checks platform type and
  calls appropriate core pll disable function.

v2: Fixed Jani's review comments.

v3: Rebased on latest drm-nightly branch.

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>