drm/radeon: use the new drm helpers for dp aux

Switch to the new dp helpers.  The main difference is
that the DP helpers don't allow an adjustable delay in
the aux transaction, but I don't know that this is
necessary.

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>