radeonfb: speed up the I2C buses

There is no reason to drive the radeon I2C buses at such a low speed, so bump
it from 12.5 kbps to 50 kbps.  The Intel (i810) and Matrox framebuffer drivers
already run their I2C bus at this speed.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: "Antonino A. Daplas" <adaplas@pol.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>