drm/amdgpu: cleanup init power gating function

Remove gfx_v9_0_enable_sck_slow_down_on_power_up/down and CP power gating
enabling functions because they only need to be called on setting power gating
behavior. We keep it in set_powergating callback to enable/disable PG in
late_init.

Signed-off-by: Huang Rui <ray.huang@amd.com>
Acked-by: Hawking Zhang <Hawking.Zhang@amd.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>