drm/i915/selftests: Assign the intel_runtime_pm pointer for mock_uncore

Couple up our mock_uncore to know about the fake global device and its
runtime powermanagement.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Matthew Auld <matthew.william.auld@gmail.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191008145045.23157-1-chris@chris-wilson.co.uk