freedreno/fdl: Don't enable r8g8 special case for mutable images

The blob seems to always use the R16 pitch align. I don't check the
block size, because the blob seems to disable UBWC when setting the
MUTABLEEN bit.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/32615>

src/freedreno/fdl/fd6_layout.c

@@ -19,7 +19,8 @@ static bool
 is_r8g8(const struct fdl_layout *layout)
 {
    return layout->cpp == 2 &&
-          util_format_get_nr_components(layout->format) == 2;
+          util_format_get_nr_components(layout->format) == 2 &&
+          !layout->is_mutable;
 }
 
 void