drm/i915: display pll hw state readout and checking

Currently still with an empty register state, this will follow in a
next step. This one here just creates the new vfunc and uses it for
cross-checking, initial state takeover and the dpll assert function.

And add a FIXME for the ddi pll readout code, which still needs to be
converted over.

v2:
- Add some hw state readout debug output.
- Also cross check the enabled crtc counting.

Note that I've botched up the patch ordering, and before this patch
we've read out the pll selection correctly, but did not reconstruct
the refcounts properly. See the bug link.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65673


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