gallium: add PIPE_SHADER_CAP_MAX_OUTPUTS and use it in st/mesa

gallium: add PIPE_SHADER_CAP_MAX_OUTPUTS and use it in st/mesa

With 5 shader stages and various combinations of enabled and disabled shaders,
the maximum number of outputs in one shader doesn't have to be equal to
the maximum number of inputs in the following shader.

v2: return 32 for softpipe and llvmpipe