drm/radeon: enable DPM by default on evergreen asics

Seems to be stable on them.  There are still some issues
with the performance states staying in the highest levels
on certain cards when multiple monitors are attached, but
being that the the cards are always in their highest power
state at boot up anyway, this doesn't really change anything
and improves things in all other cases.

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