drm/i915: lock down pch pll accouting some more

Before I start to make a complete mess out of this, crank up
the paranoia level a bit.

v2: Kill the has_pch_encoder check in put_shared_dpll - it's invalid
as spotted by Ville since we currently only put the dpll when we
already have the new pipe config. So a direct pch port -> cpu edp
transition will hit this.

v3: Now that I've lifted my blinders add the WARN_ON Ville requested.

Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>