drm/amdgpu: remove the access of xxx_PSP_DEBUG on cycan_skillfish

It won't need to clear the xxx_PSP_DEBUG registers, because firmware
will handle this change.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>