drm/i915: Propagate failure to set mode for load-detect pipe

Check the return value from drm_crtc_set_mode(), report the failure
via a debug message and propagate the error back to the caller. This
prevents us from blissfully continuing to do the load detection on a
disabled pipe. Fortunately actual failure for modesetting is very rare,
and reported failures even rarer.

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