drm/amdgpu: Write masked value to control register

On the dce_v6_0 and dce_v8_0 hpd tear down callback, the tmp variable
should be written into the control register instead of 0.

Reviewed-by: André Almeida <andrealmeid@igalia.com>
Signed-off-by: Maíra Canal <mairacanal@riseup.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>