drm/i915/selftests: Remove accidental serialization between gpu_fill

Upon object creation for live_gem_contexts, we fill the object with
known scratch and flush it out of the CPU cache. Before performing the
GPU fill, we don't need to flush it again and so avoid serialising with
previous fills.

However, we do need some throttling on the internal interfaces if we do
not want to run out of memory!

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/20190827161726.3640-1-chris@chris-wilson.co.uk