drm/i915: Don't pretend we can calculate multiple pipe_configs

The code in intel_modeset_pipe_config() still needs changes before it
can calculate more than just one pipe_config, and pretending it can will
only make those changes more difficult.

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>