drm/i915/selftests: Avoid warning if runtime pm is disabled

Inside the mock GEM device, we try to grab the runtime pm for the fake
device to prevent it from ever suspending. However, if CONFIG_PM is not
set, trying to obtain the wakref returns an error which we WARN about.
Suppress the expected warning.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180706205947.11209-1-chris@chris-wilson.co.uk