llvmpipe/format: fix snorm conversion

This fixes:
GTF-GL45.gtf33.GL3Tests.vertex_type_2_10_10_10_rev.vertex_type_2_10_10_10_rev_conversion
and
piglit
spec/arb_texture_view/rendering-formats/clear gl_rgba8_snorm as gl_r32f

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5820>
4 files changed