r600g: Handle CONFIG regs properly

r600g: Handle CONFIG regs properly

CONFIG regs (byte offsets 0x8000-0xac00) are single state and the pipeline
must be flushed and hw idle when they are changed.  Border color regs
are in the CONFIG range and this is why a flush is required when changing
them.  CONTEXT regs (byte offset 0x28000+) are multi-state and those do
not require flushes when changing them.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>