drm/i915/selftests: Replace opencoded clflush with drm_clflush_virt_range

We occasionally see that the clflush prior to a read of GPU data is
returning stale data, reminiscent of much earlier bugs fixed by adding a
second clflush for serialisation. As drm_clflush_virt_range() already
supplies the workaround, use it rather than open code the clflush
instruction.

References: 396f5d62 ("drm: Restore double clflush on the last partial cacheline")
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/20180730075351.15569-3-chris@chris-wilson.co.uk