drm/amd/powerplay: split out those internal used swSMU APIs V2

Those swSMU APIs used internally are moved to smu_internal.h while
others are kept in amdgpu_smu.h.

V2: give a better name smu_internal.h for the place to hold
    those internal APIs

Signed-off-by: Evan Quan <evan.quan@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>