panfrost: Don't compile empty blend shaders

This is silly. They'll be masked out later, but it's still pointless, and adds
noise when grepping BIFROST_MESA_DEBUG=shaders,internal for "pan_blend" when
debugging why an app is using blend shaders.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17841>

src/gallium/drivers/panfrost/pan_context.c

@@ -179,6 +179,12 @@ panfrost_get_blend(struct panfrost_batch *batch, unsigned rti, struct panfrost_b
                 return 0;
         }
 
+        /* On all architectures, we can disable writes for a blend descriptor,
+         * at which point the format doesn't matter.
+         */
+        if (info.no_colour)
+                return 0;
+
         /* On Bifrost and newer, we can also use fixed-function for opaque
          * output regardless of the format by configuring the appropriate
          * conversion descriptor in the internal blend descriptor. (Midgard