radv: Initialize BO pointers when creating preambles.

radv: Initialize BO pointers when creating preambles.

This aims to improve the readability of this function.

Initialize the BO pointers to the queue's current pointers
at the beginning instead of in the else branches later.

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>