drm/radeon: disable dpm on rv770 by default

There seem to be stability issues on a number of cards.

bugs:
https://bugs.freedesktop.org/show_bug.cgi?id=76286
https://bugzilla.redhat.com/show_bug.cgi?id=1085785
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=741619

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: matthias.graf@st.ovqu.de
Cc: bp@alien8.de
Cc: stable@vger.kernel.org