drm/i915/selftests: exercise_ggtt may have nothing to do

When operating on the live_ggtt we have to find a usuable hole for our
test. It is possible for there to be no hole we can use, so initialise
the err to 0 for the early exit.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171115152558.31252-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>