drm/i915: add generic power management initialization

This adds intel_pm routine for generic power-related infrastructure
initialization.

v2: now that all the platform-specific stuff is initialized in one place, we
can also add back the static definitions to platform-specific functions which
we abstract now.

Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-by: Ben Widawsky <benjamin.widawsky@intel.com>
Signed-off-by: Eugeni Dodonov <eugeni.dodonov@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>