drm/i915: get rid of put_shared_dpll

Now that the pll updates are staged the put_shared_dpll function
consists only of checks that are done in check_shared_dpll_state
after a modeset too.

The changes to pll->config are overwritten by
intel_shared_dpll_commit, so this entire function is a noop.

Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>