gallivm: use nearest rounding for float->unorm24 conversion

gallivm: use nearest rounding for float->unorm24 conversion

Previously we were using truncation, which gives the correct result
only for numbers in [0.5-1.0] range (because there's no mantissa bits
to do any rounding there).
This is frequently hit (and probably only used there) when converting
fragment depth to depth format (d24s8 etc.) or otherwise dealing with
depth format.

v2: as spotted by Jose, get rid of extra type (src_type is already unsigned).

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>