drm/amd: Use amdgpu_device_should_use_aspm on navi umd pstate switching

The `program_aspm` callback is already guarded for aspm, but the
`enable_aspm` callback doesn't follow the module parameter.

Update it to use the helper `amdgpu_device_should_use_aspm`.

Reviewed-by: Lijo Lazar <lijo.lazar@amd.com>
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>