drm/i915: Flush the existing fence before GGTT read/write

Our fence management is lazy, very lazy. If the user marks an object as
untiled, we do not immediately flush the fence but merely mark it as
dirty. On the next use we have to remember to check and remove the fence,
by which time we hope it is idle and we do not have to wait.

v2: Throw away the old fence on the next ggtt_pin.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111468
Fixes: 1f7fd484

 ("drm/i915: Replace i915_vma_put_fence()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190823153944.20630-1-chris@chris-wilson.co.uk