drm/i915: Basic shared dpll support for WRPLLs

Just filing in names and ids, but not yet officially registering them
so that the hw state cross checker doesn't completely freak out about
them. Still since we do already read out and cross check
config->shared_dpll the basics are now there to flesh out the wrpll
shared dpll implementation.

The idea is now to roll out all the callbacks step-by-step and then at
the end switch to the shared dpll framework. This way hw and sw
changes are clearly separated.

Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
[imre: added const to hsw_ddi_pll_names (Damien)]
Reviewed-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>