drm/radeon/aux: fix hpd assignment for aux bus

The hpd (hot plug detect) pin assignment got lost
in the conversion to to the common i2c over aux
code.  Without this information, aux transactions
do not work properly.  Fixes DP failures.

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