dma-buf: cleanup dma_fence_unwrap selftest v2

The selftests, fix the error handling, remove unused functions and stop
leaking memory in failed tests.

v2: fix the memory leak correctly.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20220518135844.3338-2-christian.koenig@amd.com