drm/amdgpu: disentangle HG systems from vgaswitcheroo

There's no need to keep vgaswitcheroo around for HG
systems.  They don't use muxes and their power control
is handled via ACPI.

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