drm/radeon: disable dpm on BTC

Still unstable on some boards.

Bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=73053
https://bugzilla.kernel.org/show_bug.cgi?id=68571



Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: 3.13 <stable@vger.kernel.org> # 3.13