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: 132b69c4 ("st/mesa: round lod_bias to a multiple of 1/256")