radeonsi: fix border color translation for integer textures

radeonsi: fix border color translation for integer textures

This fixes the extremely unlikely case that an application uses
0x80000000 or 0x3f800000 as border color for an integer texture and
helps in the also, but perhaps slightly less, unlikely case that 1 is
used as a border color.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Tested-by: Dieter Nützel <Dieter@nuetzel-hh.de>