panfrost: Enable PIPE_CAP_VERTEX_COLOR_UNCLAMPED

This tells Mesa to clamp vertex colours in the vertex shader.

This improves rendering in a number of games such as Extreme Tux
Racer and H-Craft Championships.

Cc: 20.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5075>

src/gallium/drivers/panfrost/pan_screen.c

@@ -105,6 +105,7 @@ panfrost_get_param(struct pipe_screen *screen, enum pipe_cap param)
         case PIPE_CAP_MIXED_COLOR_DEPTH_BITS:
         case PIPE_CAP_FRAGMENT_SHADER_TEXTURE_LOD:
         case PIPE_CAP_VERTEX_SHADER_SATURATE:
+        case PIPE_CAP_VERTEX_COLOR_UNCLAMPED:
         case PIPE_CAP_POINT_SPRITE:
                 return 1;