drm/i915/selftest: allow larger memory allocation

Due to changes in allocator, the size of the allocation for
contiguous region is not rounded up to a power-of-two and
instead allocated as is. Thus, change the part of test that
expected the allocation to fail.

Signed-off-by: Mikolaj Wasiak <mikolaj.wasiak@intel.com>
Reviewed-by: Krzysztof Karas <krzysztof.karas@intel.com>
Reviewed-by: Andi Shyti <andi.shyti@linux.intel.com>
Reviewed-by: Krzysztof Niemiec <krzysztof.niemiec@intel.com>
Signed-off-by: Andi Shyti <andi.shyti@linux.intel.com>
Link: https://lore.kernel.org/r/fptevdfbclvg2lbfuys5ibffbl2baouywkutnr7vdsy5tzcqfk@mpflwlh6jxfd