drm/i915: Clean-up bonding debug message.

We should stop using the gen name and the "+" to reference
the newer platforms.
And on this case specifically we can simplify the debug
message even further.

Cc: Jani Nikula <jani.nikula@intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211015091129.83226-1-rodrigo.vivi@intel.com