st_glsl_to_tgsi: bump index back up to 32-bit

st_glsl_to_tgsi: bump index back up to 32-bit

with some of the fp64 emulation, we are seeing shaders coming in with
> 32K temps, they go out with 40 or so used, but while doing register
renumber we need to store a lot of them.

So bump this fields back up to 32-bit.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>