drm/i915/icl: Add get config functionality for DSI

This patch implements the functionality for getting PIPE configuration
to which DSI encoder is connected. Use the same method to get port clock
like other DDI encoders. Used during the atomic modeset.

v2 by Jani:
- Squash Madhav's and Vandita's get config bits together
- Move cnl_calc_wrpll_link() to intel_drv.h
- Drop extra temp variables
- Use enc_to_intel_dsi() instead of open coding

Co-developed-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
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/f21fa4258068d04582f2bf30735e5536a8043bdf.1543500286.git.jani.nikula@intel.com