drm/i915: Fix up FORCE_GPU_RELOC (debug) to flush CPU write domains

We currently assert that if the target is in a CPU write domain, we use
a CPU reloc path rather than the GPU reloc path. However, we have a debug
override to force the GPU path and that unfortunately hits the assert.
Include the async clflush under the debug option to ensure correct
behaviour even when debugging, and strict when not.

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/20180903150216.19965-1-chris@chris-wilson.co.uk