vaapisink: improve check for raw YUV format mode.

vaapisink: improve check for raw YUV format mode.

Improve check for raw YUV format modes by avoiding checks against strings
("video/x-raw-yuv") for each new GstBuffer allocation. In the usual case,
GstBaseSink::set_caps() is called first and if VA surface format mode is
used, then GstBaseSink::buffer_alloc() is not called. If the latter is
called before set_caps(), then we just make a full check. This one is
pretty rare though, e.g. it usually happens once for custom pipelines.