drm/i915: hw state readout and cross-checking for shared dplls

Just the plumbing, all the modeset and enable code has not yet been
switched over to use the new state. It seems to be decently broken
anyway, at least wrt to handling of the special pixel mutliplier
enabling sequence. Follow-up patches will clean up that mess.

Another missing piece is more careful handling (and fixup) of the fp1
alternate divisor state. The BIOS most likely doesn't bother to
program that one to what we expect. So we need to be more careful with
comparing that state, both for cross checking but also when checking
for dpll sharing when acquiring shared dpll. Otherwise fastboot will
deny a few shared dpll configurations which would otherwise work.

v2: We need to memcpy the pipe config dpll hw state into the pll, for
otherwise the cross-check code will get angry.

v3: Don't forget to read the pch pll state in the crtc get_pipe_config
function for ibx/ilk platforms.

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