draw,gallivm: Fix draw_get_shader_param.

draw,gallivm: Fix draw_get_shader_param.

- Use LLVM limits when LLVM is being used, instead of TGSI limits
- Provide draw_get_shader_param_no_llvm for when llvm is never used (softpipe)
- Eliminate several of the hacks around draw shader caps in several drivers

Unfortunately the hack for PIPE_MAX_VERTEX_SAMPLERS is still necessary.

Reviewed-by: Marek Olšák <maraeo@gmail.com>