v3dv: add the concept of a job

v3dv: add the concept of a job

As we make progress towards more complex submissions we will need to split
our command buffers into smaller executable units (jobs) that we can
submit indepdently to the kernel. This will be required to implement
pipeline barriers, split subpasses that have depedencies on previous
subpasses, split render passes that use more than 4 render targets, etc.

For now we keep things simple and we only keep one job as current
recording target in the command buffer, and we generate a new one
with every subpass or with any commands we see outside of a render pass
(only vkCopyImageToBuffer for now). In the future we probably want to
optimize this by merging subpasses into the same job when possible,
etc.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6766>