radeon: fix some potential big endian issues

radeon: fix some potential big endian issues

The formats chosen (both by texture format choser, fbo storage allocation)
are different for big endian not just for rgba8 but also lower bit width
formats (why I don't actually know). Even the function to test for renderable
formats used different formats, however the actual colorbuffer setup did not.
And the blitter did not take that into account neither.
Untested (what could possibly go wrong...).

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