drm/i915: move pm related declarations to intel_pm.h

Move more missed declarations from i915_drv.h to intel_pm.h.

Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190606122203.13416-1-jani.nikula@intel.com