drm/amdgpu: Rename set_mmhub_powergating_by_smu to powergate_mmhub

In order to keep consistent with powergate_uvd/vce.

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