dma-buf: keep the signaling time of merged fences v3

Some Android CTS is testing if the signaling time keeps consistent
during merges.

v2: use the current time if the fence is still in the signaling path and
the timestamp not yet available.
v3: improve comment, fix one more case to use the correct timestamp

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230630120041.109216-1-christian.koenig@amd.com