drm/i915/icl: Get DSI transcoder for a given port

This patch adds a helper function to retrieve DSI
transcoder for a given DSI port using newly defined
enum names for DSI transcoders.

Signed-off-by: Madhav Chauhan <madhav.chauhan@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/f88ff26fa10c68e37b7838bb7c8573c881474e73.1539613303.git.jani.nikula@intel.com