drm/i915/dp: make DSC usage logging actually useful

Debug log when DSC is going to be used, and why, instead of
unconditionally logging the rarely used debug option setting, which
might not have any bearing on whether DSC is going to be used or not.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220330093019.4150386-1-jani.nikula@intel.com