drm/i915: tweak language in fastset pipe config compare logging

The "fastset mismatch" debug logging has been slightly confusing,
leading people to believe some error happened. Change it to the more
informative "fastset requirement not met", and add a final message about
this leading to full modeset.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Suraj Kandpal <suraj.kandpal@intel.com>
Acked-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230516094406.2675307-1-jani.nikula@intel.com