radeonfb: give i2c buses nicer names

The name of the i2c buses shows in the output of "i2cdetect -l", so
it's important to give informative names.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>