igt/gem_close_race: Trim object size

igt/gem_close_race: Trim object size

The challenge here is to race GPU activity versus the gem_close. Using a
large objects makes the window of GPU activity larger - except on !llc
systems we then incur massive overhead from clflush, likely destroying
any race (due to mutex contention). And that mutex contention is
amplified by the number of cores - bad news for Baytrail.

Give up and make the objects smaller and hope that the test is run
frequently enough to catch the race.

References: https://bugs.freedesktop.org/show_bug.cgi?id=71029
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>