drm/i915: Specify when the PLL hw state fields are valid

Not all those fields are valid on a given platform. Make it explicit.

Unions could also be used, but were cluttering some code paths with
if/else ladders.

v2: Don't use anonymous unions (Daniel)

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