drm/radeon: don't set default clocks for SI when DPM is disabled

This is a partial revert of c6cf7777.

We need to take into account the clk voltage dependencies of the
board.  Not doing so can lead to stability issues on certain
boards if the clks exceed the levels in the dep tables.

DPM already takes that into account, so for optimal performance,
use DPM.

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