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

drivers/gpu/drm/i915/selftests/intel_memory_region.c

@@ -352,7 +352,9 @@ static int igt_mock_splintered_region(void *arg)
 	return err;
 }
 
+#ifndef SZ_8G
 #define SZ_8G BIT_ULL(33)
+#endif
 
 static int igt_mock_max_segment(void *arg)
 {