drm/i915: Preserve shared DPLL information in new pipe_config

When a new pipe_config is calculated, the fields related to shared dplls
are reset, under the assumption that they will be recalculated as part
of the modeset, which is true with the current state of the code.

As we convert to atomic, however, it will be possible to calculate a new
pipe_config and skip the modeset. In that case, after the state swap we
still want the shared dplls to be preserved.

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>