drm/i915: move some leftovers to intel_pm.h from i915_drv.h

Commit 696173b0 ("drm/i915: extract intel_pm.h from intel_drv.h")
missed the declarations in i915_drv.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/770f5f1c2dd99e4d6a314b70184e71b928a6d362.1556540890.git.jani.nikula@intel.com