drm/amd: Explicitly disable ASPM when dynamic switching disabled

Currently there are separate but related checks:
* amdgpu_device_should_use_aspm()
* amdgpu_device_aspm_support_quirk()
* amdgpu_device_pcie_dynamic_switching_supported()

Simplify into checking whether DPM was enabled or not in the auto
case.  This works because amdgpu_device_pcie_dynamic_switching_supported()
populates that value.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>