drm/i915/icl: add pll mapping for DSI

Add encoder specific pll mapping for DSI. The differences with the DDI
version are big enough to warrant a separate function.

v2: add posting read (Madhav)

Cc: Madhav Chauhan <madhav.chauhan@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181203094326.28294-1-jani.nikula@intel.com