v3dv: pre-allocate actual events instead of event descriptors

v3dv: pre-allocate actual events instead of event descriptors

Instead of keeping a free list of "event descriptors" which are
just the offsets in the BO state that are available, pre-allocate
the events. This is simpler as it doesn't require to allocate these
event descriptors at all.

Reviewed-by: Alejandro Piñeiro <apinheiro@igalia.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20053>