drm/i915/icl: Add DSI encoder compute config hook

This patch implements compute config for Gen11 DSI encoder which is
required at the time of modeset.

For DSI 8X clock is AFE clock which is 5 times port clock.

v2 by Jani:
 - drop the enable nop hook
 - fixed_mode is always true
 - HAS_GMCH_DISPLAY() is always false

v3 by Jani:
 - set encoder->compute_config dropped during rebase

v4 by Jani:
 - squash Vandita's port clock patch
 - remove todo comment

Co-developed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Signed-off-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a21574173caa5e2932d9e3c537b0931097ab5ac2.1543500286.git.jani.nikula@intel.com