drm/amdgpu: init atpx at switcheroo register time (v2)

If we do it at enable time, it's too late for the feature
checks.

v2: drop .init setting as per Peter's comments

Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: Peter Wu <peter@lekensteyn.nl>