VA drivers allocate surfaces given their properties, so there's no need to
provide a buffer size to the VA pool.

Though, the buffer size is provided by the driver, or the canonical size
is used for single planed surfaces.

This patch removes the need to provide a size for the function
gst_va_pool_new_with_config() and adds a helper method to retrieve the surface
size, gst_va_pool_get_buffer_size(). Also change the callers accordingly.

Changes for custom VA pool creation will be addressed in the following commits.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/5805>