drm/i915: Wait for all fences before installing an exclusive clflush fence

drm/i915: Wait for all fences before installing an exclusive clflush fence

Ensure that before we overwrite the reservation_object with our
exclusive fence for the pending clflush operation, that we do wait upon
all the fences in the current reservation_object.

Fixes: 57822dc6b9cf ("drm/i915: Perform object clflushing asynchronously")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170323085758.11695-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>