drm/amdgpu/powerplay: program display gap for tonga.

Implement displaygap programming for tonga.  This is
required for properly mclk switching.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Jammy Zhou <Jammy.Zhou@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>