drm/i915: Update load-detect failure paths for modeset-rework

After the rework, intel_set_mode() became a little better behaved in
restoring the current mode if we failed to apply the requested modeline.
However, the failure path for load-detect would clobber the existing
state, leading to an oops during BIOS takeover on older machines.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>