r200: fix fbo rendering by disabling optimized texture format chooser

r200: fix fbo rendering by disabling optimized texture format chooser

It is rather unfortunate that we don't know if a texture is going to be used
as a rt later, and we lack the means to do something about a format chosen
which we can't render to directly, so disable this and always chose renderable
format for rgba8 textures.
This addresses an issue raised on (old) bug,
https://bugs.freedesktop.org/show_bug.cgi?id=51658 with gnome-shell, don't
know if that's still applicable but it might fix other things as well.

Acked-by: Marek Olšák <marek.olsak@amd.com>