radeonsi: process TGSI property NEXT_SHADER

radeonsi: process TGSI property NEXT_SHADER

This allows compiling the main shader part as ES or LS.

If we get the correct hint, non-separable GLSL shaders no longer have to be
compiled as VS first, followed by LS or ES compiled on demand.

The result is that fewer shaders are compiled by piglit, but it doesn't
improve piglit running time.

Reviewed-by: Nicolai Hähnle <nicolai.haehnle@amd.com>