v3dv: fix leaks during recording of meta blits

v3dv: fix leaks during recording of meta blits

This uses the framework to register private commmand buffer objects
that get freed automatically when the command buffer is destroyed by
the application.

This change also moves the descriptor set pool that the meta blit path
uses to allocate descriptors for the blit source textures, from the
device to the command buffer, so we can have a descriptor pool per
command buffer.  This is necessary to ensure correct behavior when
doing multi-threaded command buffer recording (alternatively, we would
have to lock around the descriptor set allocation code, which would be
undesirable).

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