drm/radeon/dpm/atom: restructure logic to work around a compiler bug

It seems gcc 4.8.1 generates bogus code for the old logic causing
part of the function to get skipped.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=66932
https://bugs.freedesktop.org/show_bug.cgi?id=66972
https://bugs.freedesktop.org/show_bug.cgi?id=66945

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>