drm/i915: Demote drop_caches_set print

Many tests call this ad naseum now (in an infinite loop, very often).
It clutters the logs. Actually, I'd rather drop it completely...

Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Acked-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>