vaapi: decoder: modify the condition to judge whether dma buffer is supported

vaapi: decoder: modify the condition to judge whether dma buffer is supported

It seems "GST_VAAPI_PLUGIN_BASE_SRC_PAD_CAN_DMABUF (decode)" will
return false even if this platform support the mem_type dma buffer.
And media-driver will return GST_VAAPI_BUFFER_MEMORY_TYPE_DMA_BUF2
on Gen12(such as TGL).
Without this patch, The command such as:
gst-launch-1.0 videotestsrc num-buffers=100 ! video/x-raw, format=I420 ! \
x264enc ! h264parse ! vaapih264dec ! video/x-raw\(memory:DMABuf\) ! fakesink
will return not-negotiated.

Signed-off-by: Zhang Yuankun <yuankunx.zhang@intel.com>
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer-vaapi/-/merge_requests/437>