swr: mark color clamping as unsupported

There is no functionality in swr to clamp either vertex or frag colors.
This could be added in swr_shader, at which point these could be
re-enabled.

Fixes arb_color_buffer_float-render

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Tim Rowley <timothy.o.rowley@intel.com>

src/gallium/drivers/swr/swr_screen.cpp

@@ -233,8 +233,9 @@ swr_get_param(struct pipe_screen *screen, enum pipe_cap param)
    case PIPE_CAP_TEXTURE_BARRIER:
       return 0;
    case PIPE_CAP_FRAGMENT_COLOR_CLAMPED:
-   case PIPE_CAP_VERTEX_COLOR_UNCLAMPED: /* draw module */
-   case PIPE_CAP_VERTEX_COLOR_CLAMPED: /* draw module */
+   case PIPE_CAP_VERTEX_COLOR_CLAMPED:
+      return 0;
+   case PIPE_CAP_VERTEX_COLOR_UNCLAMPED:
       return 1;
    case PIPE_CAP_MIXED_COLORBUFFER_FORMATS:
       return 1;