v3dv: implement host-side event handling functions

v3dv: implement host-side event handling functions

I am not quite certain that this is the way to go though. Here, we are
expecting that the GPU can set/reset the event inside a command buffer
as a 1x1 pixel clear for example, however, there is still the question
of how we get to implement the command buffer wait on an event, since
reading the docs I haven't found any such functionality to be available.
We could think of implementing this by splitting the command buffer
into multiple jobs at the wait command, and then using a separate
thread for job submissions that would poll the event UBO before sending
it to the kernel, but that looks like a bit of a kludge.

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