selftests: dmabuf-heap: add config file for the test

The config fragment enlists all the config options needed for the test.
This config is merged into the kernel's config on which this test is
run.

Fixed whitespace errors during commit:
Shuah Khan <skhan@linuxfoundation.org>

Reviewed-by: T.J. Mercier <tjmercier@google.com>
Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>