gallium/st: correct range for int16
This fixes the following dEQP tests:
- dEQP-GLES3.functional.shaders.operator.unary_operator.minus.lowp_uvec2_fragment
- dEQP-GLES3.functional.shaders.operator.unary_operator.minus.lowp_uvec4_fragment
- dEQP-GLES3.functional.shaders.operator.unary_operator.minus.mediump_uvec2_fragment
- dEQP-GLES3.functional.shaders.operator.unary_operator.minus.mediump_uvec4_fragment
Fixes:
1af8fe4ed52 ("gallium: add shader caps INT16 and FP16_DERIVATIVES")
Reviewed-by: Alyssa Rosenzweig <alyssa@collabora.com>
Tested-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10128>