drm/i915/gt: Force pte cacheline to main memory

We have problems of tgl not seeing a valid pte entry when iommu is
enabled. Add heavy handed flushing of entry modification by flushing the
cpu, cacheline and then wcb. This forces the pte out to main memory past
this point regarless of promises of coherency.

This is an evolution of an experimental patch from Chris Wilson of adding
wmb for coherent partners, by adding a clflush to force the cache->memory
step.

Closes: https://gitlab.freedesktop.org/drm/intel/-/issues/1840


Testcase: igt/gem_exec_fence/parallel
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20200511160803.15407-1-mika.kuoppala@linux.intel.com