drm/i915/selftests: Grab the runtime pm in shrink_thp

Since we are not holding a wakeref, shrinking a bound object is not
guaranteed.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/6370


Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220706154738.235204-1-matthew.auld@intel.com