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