drm/i915: Skip dsc readout if the transcoder is disabled

Trying to do readout when we don't even have a cpu transcoder
is not a great idea. Don't do it.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220124192638.26262-1-ville.syrjala@linux.intel.com


Reviewed-by: Jani Nikula <jani.nikula@intel.com>