radeon/r200/r300: fix up the whole buffer space checking.

radeon/r200/r300: fix up the whole buffer space checking.

This fixes up the buffer validation scheme, so that we keep a list
of buffers to validate so cmdbuf flushes during a pipeline get
all the buffers revalidated on the next emit.

This also fixes radeonFlush to not flush unless we have something
useful to send to the GPU, like a DMA buffer or something not state