st/mesa: Invalidate the gallium array atom only if needed.

st/mesa: Invalidate the gallium array atom only if needed.

Now that the buffer object usage history tracks if it is
being used as vertex buffer object, we can restrict setting
the ST_NEW_VERTEX_ARRAYS bit to dirty on glBufferData calls to
buffers that are potentially used as vertex buffer object.
Also put a note that the same could be done for index arrays
used in indexed draws.

Reviewed-by: Brian Paul <brianp@vmware.com>
Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>