radv: Use radv_fill_buffer for accel struct builds

radv: Use radv_fill_buffer for accel struct builds

It turns out, that the fuchsia sort may actually
perform clears with size < 16 which hits an assert
in radv_fill_buffer_shader. This fixes random
crashes in Control.

Fixes: be57b08 ("radv: Build accaleration structures using LBVH")
Signed-off-by: Konstantin Seurer <konstantin.seurer@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/16517>