va: pool: simplify the logic

va: pool: simplify the logic

Instead of removing memories from buffers at reset_buffer()/release_buffer() the
bufferpool operation is kept as originally designed, still the allocator pool is
used too. Thus, this patch restores the buffer size configuration while removing
release_buffer(), reset_buffer() and acquire_buffer() vmethods overloads.

Then, when the bufferpool base class decides to discard a buffer, the VA
surface-based memory is returned to the allocator pool when its last reference
is freed, and later reused if a new buffer is allocated again.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2013>