drm/amd/pm/si: Notify the SMC when switching to AC

drm/amd/pm/si: Notify the SMC when switching to AC

There are some platforms that don't have a dedicated
GPIO line to manage the AC/DC switch. In this case,
the SI SMC automatically notices when switching to DC,
but needs to be notified when switching to AC.

Fixup and use si_notify_hw_of_powersource() which was
previously hidden behind an "#if 0".

This fixes some SI laptop GPUs to be able to use their
performance power states after switching from DC to AC.

Some affected GPUs are:
FirePro W4170M - Dell Precision M2800
Radeon HD 8790M - Dell Latitude E6540

Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Co-developed-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Signed-off-by: Jeremy Klarenbeek <jeremy.klarenbeek99@gmail.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>