drm/i915: make intel_wakeref work on the rpm struct

intel_runtime_pm is the only thing they use from the i915 structure,
so use that directly.

Signed-off-by: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190613232156.34940-9-daniele.ceraolospurio@intel.com