r300: explicitly advertise index buffer format support

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11132>

src/gallium/drivers/r300/r300_screen.c

@@ -652,6 +652,13 @@ static bool r300_is_format_supported(struct pipe_screen* screen,
         }
     }
 
+    if (usage & PIPE_BIND_INDEX_BUFFER) {
+       if (format == PIPE_FORMAT_R8_UINT ||
+           format == PIPE_FORMAT_R16_UINT ||
+           format == PIPE_FORMAT_R32_UINT)
+          retval |= PIPE_BIND_INDEX_BUFFER;
+    }
+
     return retval == usage;
 }