drm/i915: Don't store temporary load-detect variables in the generic encoder

Keep all the state required for undoing and restoring the previous pipe
configuration together in a single struct passed from
intel_get_load_detect_pipe() to intel_release_load_detect_pipe() rather
than stuffing them inside the common encoder structure.

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>