v3dv: handle barriers at the end of a command buffer

v3dv: handle barriers at the end of a command buffer

Since we only consume barriers at the beginning of a new job, if
a command buffer ends with a barrier we will not handle it. Fix
this by emitting a noop job  in that case to consume it. Ideally,
we could do better and check the pending barrier state to fine
tune the noop job so we don't wait on all queues, but for now
this fixes flakyness with some CTS pipeline barrier tests that
started to show up after we optimized binning sync barriers. It
is likely that the additional sync we had before that change was
enough to prevent the problem from showing up.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17020>