drm/i915/ringbuffer: Delay after invalidating gen6+ xcs

During stress testing of full-ppgtt (on Baytrail at least), we found
that the invalidation around a context/mm switch was insufficient (writes
would go astray). Adding a second MI_FLUSH_DW barrier prevents this, but
it is unclear as to whether this is merely a delaying tactic or if it is
truly serialising with the TLB invalidation. Either way, it is
empirically required.

v2: Avoid the loop for readability;

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=107715
References: https://bugs.freedesktop.org/show_bug.cgi?id=107759
Signed-off-by: Chris Wilson <[email protected]>
Cc: Joonas Lahtinen <[email protected]>
Cc: Mika Kuoppala <[email protected]>
Cc: Matthew Auld <[email protected]>
Cc: Tvrtko Ursulin <[email protected]>
Reviewed-by: Ville Syrjälä <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

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