drm/i915: Don't use plane update helper in legacy mode set

Use lower level calls to better integrate with the modeset code and
allow a full state swap in a follow up patch.

Signed-off-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>