evas: Avoid more calls to efl_isa in clip_set

evas: Avoid more calls to efl_isa in clip_set

No need to verify that the clip is an evas object unless we're
actually going to change said clipper. This has a minor effect,
removing ~10% of all calls to efl_isa (0.1% of everything).

After all those efl_isa patches the number of calls has dropped
from 1.9M (~3.05%) to ~570k (0.76%). More can still be done,
surely. Now efl_isa accounts for ~0.05% of my CPU usage according
to perf top (down from ~0.7%).

If edje uses EO APIs instead of legacy image APIs, then many of
the remaining calls will vanish.

@optimization