va: allocator: create surfaces with VASurfaceAttribExternalBuffers

va: allocator: create surfaces with VASurfaceAttribExternalBuffers

Add a new parameter to _create_surfaces(): a pointer to
VASurfaceAttribExternalBuffers.

If it's defined the memory type is changed to DRM_PRIME, also a new item is
added to the VASurfaceAttrib array with
VASurfaceAttribExternalBufferDescriptor.

Also, the VASurfaceAttrib for pixel format is not mandatory anymore. If fourcc
parameter is 0, is not added in the array, relying on the chroma. This is
useful when creating surfaces for uploading or downloading images.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1529>