drm/i915: Remove dead code from intel_get_load_detect_pipe()

As we only allow the use of a disabled CRTC, we don't need to handle the
case where we are reusing an already enabled pipe.

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