drm/amdgpu: move atom scratch register save/restore to common code

We need this for more than just DCE.  Move it out of the DCE modules
and into the device code.  This way we can be sure the scratch registers
are initialized properly before we run asic_init which happens before
DCE IPs are restored.

Fixes atombios hangs in asic_init.

Reviewed-by: JimQu <Jim.Qu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>