drm/i915/icl: Enable DSI transcoders

This patch enables DSI transcoders by writing to
TRANS_CONF registers and wait for its state to be enabled.

v2 by Jani:
 - Rebase

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/4b8ea0298ef9d6832a2dd69c923832d0b7b58184.1539613303.git.jani.nikula@intel.com