drm/i915/icl: Consider DSI for getting transcoder state

For Gen11 DSI, we use similar registers like for eDP
to find if DSI encoder is connected or not to a pipe.
This patch refactors existing hsw_get_transcoder_state()
to handle this.

v2 by Jani:
 - Add WARN_ON(dsi && edp) (Ville)

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/2e10b977dfc7aa985a8559d6cd59ed0981848e95.1543500286.git.jani.nikula@intel.com