radeonsi: fix vertex fetches for 2_10_10_10 formats

radeonsi: fix vertex fetches for 2_10_10_10 formats

The hardware always treats the alpha channel as unsigned, so add a shader
workaround. This is rare enough that we'll just build a monolithic vertex
shader.

The SINT case cannot actually happen in OpenGL, but I've included it for
completeness since it's just a mix of the other cases.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>