drm/i915/selftests: Guard against redifinition of SZ_8G

In the near future, upstream will introduce a SZ_8G macro that is
slightly different to our own. Employ a temporary ifndef to avoid
compilation failure until we have backmerged.

References: 8b0fac44 ("sizes.h: add SZ_8G/SZ_16G/SZ_32G macros")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210104171511.32684-1-chris@chris-wilson.co.uk