drm/i915/icl: Get HW state for DSI encoder

This patch read out the current hw state for DSI and
return true if encoder is active.

v2 by Jani:
 - Squash connector get hw state hook here
 - Squash encode get hw state fix here

v3 by Jani:
 - Add encoder->get_power_domains() (Imre)

v4 by Jani:
 - Make encoder->get_power_domains() sensible... (Imre)

v5 by Jani:
 - Power domains are bit positions, not bits (Stan, Imre)

Cc: Imre Deak <imre.deak@intel.com>
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/ec6da32a55b9fb045527f14e41ed3dce86d46a97.1543500286.git.jani.nikula@intel.com