drm/amdgpu: Avoid leaking PM domain on driver unbind (v2)

We only support vga_switcheroo and runtime pm on PX/HG systems
so forcing runpm to 1 doesn't do anything useful anyway.

Only call vga_switcheroo_init_domain_pm_ops() for PX/HG so
that the cleanup path is correct as well.  This mirrors what
radeon does as well.

v2: rework the patch originally sent by Lukas (Alex)

Acked-by: Lukas Wunner <lukas@wunner.de>
Reported-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Lukas Wunner <lukas@wunner.de> (v1)
Cc: stable@vger.kernel.org