st/mesa: set ctx->Const.MaxSamples = 0, not 1

The gallium docs for pipe_screen::is_format_supported() says that
samples==0 or samples==1 both mean that multisampling is not supported.
Return GL_MAX_SAMPLES==0 instead of 1 for consistency with other drivers.

Note: This is a candidate for the 9.0 branch.
Reviewed-by: Marek Olšák <maraeo@gmail.com>

src/mesa/state_tracker/st_extensions.c

@@ -653,6 +653,10 @@ void st_init_extensions(struct st_context *st)
          break;
       }
    }
+   if (ctx->Const.MaxSamples == 1) {
+      /* one sample doesn't really make sense */
+      ctx->Const.MaxSamples = 0;
+   }
 
    if (ctx->Const.MaxDualSourceDrawBuffers > 0)
       ctx->Extensions.ARB_blend_func_extended = GL_TRUE;