drm/i915: Squelch nop wait-for-idle trace

If the system is already idle, omit the GEM_TRACE saying we are about to
wait for idle. It looks confusing in the logs to see a continual stream
of wait-for-idle, as one immediately assumes it is stuck in a loop.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Stuart Summers <stuart.summers@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723091218.5886-1-chris@chris-wilson.co.uk