v3d: Don't forget to bump the number of writes when doing TFU ops.

generatemipmap is just filling out the rest of the mipmap that's already
been written (by a mapping or a draw call), so it didn't matter.  As I
reuse the TFU code for linear-to-UIF conversions, it'll start mattering.

src/gallium/drivers/v3d/v3d_blit.c

@@ -418,6 +418,8 @@ v3d_generate_mipmap(struct pipe_context *pctx,
                 return false;
         }
 
+        rsc->writes++;
+
         return true;
 }