drm/i915/skl: Move gen9 pm initialization into its own branch

Gen9 is different enough (for instance, fetching the memory latency
values is different from ILK+) to not take the HAS_PCH_SPLIT() branch,
so let's prefer a clean separation.

v2: Rebase on top of the broadwell_init_clock_gating() name change

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