drm/amdgpu/powerplay: add function point in hwmgr_funcs for program display gap

Displaygap support is required for proper 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>