drm/amdgpu: rework runtime pm enablement for BACO

Add a switch statement to simplify asic checks.  Note
that BACO is not supported on APUs, so there is no
need to check them.

Reviewed-by: Felix Kuehling <Felix.Kuehling@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>