drm/i915/selftests: Tweak igt_ggtt_page to speed it up

Reduce the number of GGTT PTE operations to speed the test up, but we
reduce the likelihood of spotting a coherency error in those operations.
However, Broxton is sporadically timing on this test, presumably because
its GGTT operations are all uncached.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171223110407.21402-1-chris@chris-wilson.co.uk