drm: Right-shift I2C slave address before passing it to aux channel

In r249041, I2C slave addresses were left-shifted at creation time to
have the same behavior between i915 and radeon (not committed yet). This
change broke the aux channel. The user-visible change was that display
port on i915 didn't work anymore.

To fix this, we right-shift the address back to restore the original value.

Reported by: Olivier Cochard-Labbé <olivier@cochard.me>
Tested by: Olivier Cochard-Labbé <olivier@cochard.me>
Reviewed by: kib@