drm/intel: refactor DP i2c support and DP common header to drm helper

Both radeon and nouveau can re-use this code so move it up a level
so they can. However the hw interfaces for aux ch are different
enough that the code to translate from mode, address, bytes
to actual hw interfaces isn't generic, so move that code into the
Intel driver.

Signed-off-by: Dave Airlie <airlied@redhat.com>