runqueue: Ensure setscene tasks with overlapping stamps don't parallel execute

runqueue: Ensure setscene tasks with overlapping stamps don't parallel execute

In multiconfig, mutliple tasks can execute which share the same stamp file. These
must not execute in parallel, the idea is the first should execute, the subsequent
ones should see a valid stamp and get skipped.

The normal task execution code has stamps code to handle this, this adds similar
code to the setscene execute() function to handle the issue there too.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>