drm/amd/display: add NULL checks for clock manager pointer

This fixes kernel NULL pointer dereferences on shutdown:
RIP: 0010:build_audio_output.isra.0+0x97/0x110 [amdgpu]
RIP: 0010:enable_link_dp+0x186/0x300 [amdgpu]

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Ahzo <Ahzo@tutanota.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>