drm/i915: Disable all planes for load detection, v2.

drm/i915: Disable all planes for load detection, v2.

We don't need any active planes during load detection, so just disable
them all. This saves us from having to come up with a suitable
framebuffer. And we also avoid leaving sprite/cursor planes on and
potentially presenting them at a peculiar location during the load
detection.

Changes since v1 (Maarten):
- Add missing call to add_all_affected_planes.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=102707
Link: https://patchwork.freedesktop.org/patch/msgid/20171220093545.613-4-maarten.lankhorst@linux.intel.com
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>