radv: Don't create continue preamble when it's not needed.

radv: Don't create continue preamble when it's not needed.

Previously we would always create it, but would return NULL when
it wasn't needed. Now, don't create it when not needed.

Additionally, don't create the continue preamble when we can use
IB BOs, because then we never use a continue preamble.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-By: Tatsuyuki Ishi <ishitatsuyuki@gmail.com>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16358>