drm/i915: Fix preallocated barrier list append

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: 841350223816 ("drm/i915/gt: Drop mutex serialisation between context pin/unpin")
Cc: Chris Wilson <[email protected]>
Cc: Maarten Lankhorst <[email protected]>
Signed-off-by: José Roberto de Souza <[email protected]>
Reviewed-by: Chris Wilson <[email protected]>
Signed-off-by: Chris Wilson <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]
(cherry picked from commit d4c3c0b8221a72107eaf35c80c40716b81ca463e)
Signed-off-by: Jani Nikula <[email protected]>