llvmpipe: fix d32 unorm depth conversions.

llvmpipe: fix d32 unorm depth conversions.

When the depth value was 1.0 and was being converted to Z32_UNORM
the conversion would scale it up to INT32_MAX + 1 which would
cause FPToSI to give incorrect results, changing it to use
FPToUI for the unsigned 32-bit case only fixes it.

Fixes:
GTF-GL45.gtf30.GL3Tests.depth_texture.depth_texture_fbo_clear

Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4574>