drm/i915/selftests: Take rpm wakeref around GGTT lowlevel tests

drm/i915/selftests: Take rpm wakeref around GGTT lowlevel tests

The vma routines are responsible for acquiring the device rpm wakeref
before they poke the HW. However, some of the selftests bypass the
higher level vma routines in order to poke directly at the lowlevel GGTT
functions; these are then responsible for managing rpm themselves.

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/20171107114051.10583-1-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>