drm/amd/pm: enable ASPM by default

Since ASPM function has been stable, we don't need to add the modprobe
parameter and we can enable ASPM by default.

Signed-off-by: Kenneth Feng <kenneth.feng@amd.com>
Reviewed-by: Jiansong Chen <Jiansong.Chen@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>