st/mesa: use roundf instead of floorf for lod-bias rounding

There's no good reason not to use a symmetric rounding mode here. This
fixes the following GL CTS case for me:

GTF-GL33.gtf21.GL3Tests.texture_lod_bias.texture_lod_bias_all

Fixes: 132b69c4edb ("st/mesa: round lod_bias to a multiple of 1/256")
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6892>
1 file changed