drm/i915: Don't store current shared DPLL in the new pipe_config

Now that shared DPLLs configuration is staged, there's no need to track
the current ones in the new pipe_config since those are released before
making the new pipe_config effective.

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