[Intel-gfx] Remove dead code from intel_get_load_detect_pipe

[Chris Wilson]

One of the original chores for intel_get_load_detect_pipe() was to steal
a CRTC from an active output for the transient purpose of load detection.
Understandably this code was removed some time ago, but its legacy remains.
In order to fix a real bug, where we failed to attach a fb, I first had to
remove those extraneous complications and so make the real bug fix (and
every step towards it) as clear and as simple as possible (I hope!).
-Chris