drm/i915: Fix preallocated barrier list append

Only the first and the last nodes were being added to
ref->preallocated_barriers.

Renaming variables to make it more easy to read.

Fixes: 84135022 ("drm/i915/gt: Drop mutex serialisation between context pin/unpin")
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: José Roberto de Souza <jose.souza@intel.com>
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/20200129232345.84512-1-jose.souza@intel.com