va: basedec: Fix a bug of src caps decision.

In the get_preferred_format_and_caps_features(), the caps query of downstream using gst_pad_get_allowed_caps() always returns a valid caps. The returned value is the intersection of downstream's sink caps and our src caps. So, for an "any caps" downstream element, such as filesink, we still get a preferred feature of VAMemory and fail to copy the data to system memory.