drm/amdgpu/smu: rework i2c adpater registration

The i2c init/fini functions just register the i2c adapter.
There is no need to call them during hw init/fini.  They only
need to be called once per driver init/fini.  The previous
behavior broke runtime pm because we unregistered the i2c
adapter during suspend.

Tested-by: Tom St Denis <tom.stdenis@amd.com>
Reviewed-by: Kevin Wang <kevin1.wang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>