drm/i915: Do not touch best_encoder for load detect.

This should only be touched by drm_atomic_helper.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1452160762-30487-4-git-send-email-maarten.lankhorst@linux.intel.com