drm/i915/dsc: clarify DSC support for pipe A on ICL

The check for cpu_transcoder != TRANSCODER_A is more magic than
necessary, and potentially misleading. Before TGL, DSC is supported on
pipe A if, and only if, it's used with eDP or DSI transcoders. No
functional changes.

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/f00e9d55ce20b256177222588780c660aa587cc3.1576081155.git.jani.nikula@intel.com