drm/i915: Remove dead code from intel_release_load_detect_pipe()

As we now never attempt to steal a crtc for load detection, we either
set a mode on a new pipe, or change the dpms mode on an existing pipe.
Never both, so we can simplify the code slightly.

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