drm/radeon/kms: add support for hw i2c on r1xx-r5xx

wire hw i2c support into radeon i2c algo.

fixes merged:
- handle bus probing correctly
- use meaningful error numbers
- abort if transaction fails
- The line mapping is different depending on the asic.
- protect hw i2c engine with a mutex
- rs300 doesn't have a pin select bit
- r200 has a different pin select setup

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>