media/rpivid: Make SPS / PPS optional in a request

SPS & PPS are optional in requests. Fix. The framework keeps the last
value so this is mostly a matter of changing .required to false when
requesting the controls. Check that SPS has ever been set on frame
start, PPS is valid if all zeros so is at best tricky to check.

Signed-off-by: John Cox <jc@kynesim.co.uk>