drm/i915/selftests: Take rpm wakeref around partial tiling tests

Since the partial tiling tests are poking into the GGTT to watch the
fence registers in operation, it itself needs the device rpm wakeref in
order for the GGTT to remain accessible.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171107115653.10716-1-chris@chris-wilson.co.uk


Reviewed-by: Matthew Auld <matthew.auld@intel.com>