radeon/r200: fix bogus clears

radeon/r200: fix bogus clears

There were several problems with these functions (which are a remnant
of dri1 hyperz mostly - should bring it back somehow someday).
First, it would always do a swrast clear if the buffer to clear was a fbo.
Second, for buffers we wouldn't handle the clear (I guess aux/accum?) we
would actually still have tried to clear that later even when we already
cleared it with swrast.