radv: Flush before copying with PKT3_WRITE_DATA in CmdUpdateBuffer

radv: Flush before copying with PKT3_WRITE_DATA in CmdUpdateBuffer

Need to flush before updating the buffer to ensure that the copy is
ordered after previous accesses (assuming the app has performed the
appropriate barriers).

This fixes potential issues due to draws prior to an update reading
the new buffer content, despite having the necessary barriers between
them.

Signed-off-by: Alex Smith <asmith@feralinteractive.com>
Cc: 17.0 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Dave Airlie <airlied@redhat.com>