drm/amdgpu: disentangle runtime pm and vga_switcheroo

Originally we only supported runtime pm on PX/HG laptops
so vga_switcheroo and runtime pm are sort of entangled.

Attempt to logically separate them.

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