drm/i915/selftests: Trim blitter block size

Reduce the amount of work we do to verify client blt correctness as
currently our 0.5s subtests takes about 15s on slower devices!

v2: Grow the maximum block size until we run out of time

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/20200210231047.810929-1-chris@chris-wilson.co.uk