drm/i915: Unconditionally flush any chipset buffers before execbuf

If userspace is asynchronously streaming into the batch or other
execobjects, we may not flush those writes along with a change in cache
domain (as there is no change). Therefore those writes may end up in
internal chipset buffers and not visible to the GPU upon execution. We
must issue a flush command or otherwise we encounter incoherency in the
batchbuffers and the GPU executing invalid commands (i.e. hanging) quite
regularly.

v2: Throw a paranoid wmb() into the general flush so that we remain
consistent with before.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90841
Fixes: 1816f9236303 ("drm/i915: Support creation of unbound wc user...")
Signed-off-by: Chris Wilson <[email protected]>
Cc: Akash Goel <[email protected]>
Cc: Daniel Vetter <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Tested-by: Matti Hämäläinen <[email protected]>
Cc: [email protected]
Reviewed-by: Mika Kuoppala <[email protected]>
Link: http://patchwork.freedesktop.org/patch/msgid/[email protected]

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