drm/i915: WARN on lack of shared dpll

Now that we have proper hw state reconstruction we should never have a
case where we don't have the software dpll state properly set up. So
add WARNs to the respective !pll cases in enable/disabel_shared_dpll.

Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>