drm/i915: Simplify return value from intel_get_load_detect_pipe

... and so remove the confusion as to whether to use the returned crtc
or intel_encoder->base.crtc with the subsequent load-detection. Even
though they were the same, the two instances of load-detection code
disagreed over which was the more correct.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Keith Packard <keithp@keithp.com>