drm/i915/icl: Define display GPIO pins for DSI

Display Pins are the only GPIOs that need to be used by
driver for DSI panels. So driver should now have its own
implementation to toggle these pins based on GPIO info
received from VBT sequences.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/e7e3bb05d4f48b1876169a69f495bcf6d511fda5.1543500286.git.jani.nikula@intel.com