va: h264dec: Set the padding to VideoAlignment's right.

va: h264dec: Set the padding to VideoAlignment's right.

In our va implemenation, we just use image's info to map the buffer.
The padding info just plays a role as a place holder to expand the
allocation size in caps when decoding size is bigger than display
size. So the padding_right or padding_left does not change the result.
But we find if using padding_left, it is hard to meet the requirement
of gst_video_meta_validate_alignment(), when the video meta's stride
is different from the allocation width.

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