drm/radeon/kms/atom: fix possible segfault in pm setup

commit 4376eee9 upstream.

If we end up with no power states, don't look up
current vddc.

fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=44130



agd5f: fix patch formatting

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>