drm/amdgpu/atom: Send out the full AUX address

AUX addresses are 20 bits long. Send out the entire address instead of
just the low 16 bits.

Port of:
drm/radeon/atom: Send out the full AUX address
to amdgpu

Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org