va: allocator: Decide the allocator's parameters when set_format().

va: allocator: Decide the allocator's parameters when set_format().

Moving the parameters testing and setting from the allocator_alloc_full()
to the allocator_try(). The allocator_alloc_full() will be called every
time when we need to allocate a new memory. But all these parameters such
as the surface and the image format, rt_format, etc, are unchanged during
the whole allocator lifetime. Just setting them in set_format() is enough.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1698>