drm/i915/dsc: fix DSC power domains for DSI

Fix several issues with DSC power domains that did not take DSI
transcoders into account:

- On TGL+ we need to use PW2 for DSC on pipe A, not transcoder A. There
  is no longer an eDP transcoder, but there are two DSI transcoders
  which may be connected to pipe A.

- On TGL+ we need to use the pipe, not transcoder, power domains for DSC
  on pipes other than A. Again, there are DSI transcoders.

- On ICL we need to use PW2 for DSC also for DSI transcoders, not just
  for the eDP transcoder.

Using is_pipe_dsc() also adds the warning about ICL pipe A DSC, which
does not exist.

Cc: José Roberto de Souza <jose.souza@intel.com>
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Vandita Kulkarni <vandita.kulkarni@intel.com>
Reviewed-by: Vandita Kulkarni <vandita.kulkarni@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191212134728.18432-1-jani.nikula@intel.com