vgaswitcheroo: switch the mux to the igp on power down when runpm is enabled

Avoids blank screens on muxed systems when runpm is active.

bug:
https://bugs.freedesktop.org/show_bug.cgi?id=75917



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org