drm/radeon/r100/r200: fix calculation of compressed cube maps

This needs similar handling to other compressed textures.
Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=26428

Signed-off-by:  <sroland@vmware.com>
Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Cc: stable <stable@kernel.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>