drm/radeon/dpm: make sure dc performance level limits are valid (BTC-SI) (v2)

Check to make sure the dc limits are valid before using them.
Some systems may not have a dc limits table.  In that case just
use the ac limits.  This fixes hangs on systems when the power
state is changed when on battery (dc) due to invalid performance
state parameters.

Should fix:
https://bugs.freedesktop.org/show_bug.cgi?id=68708



v2: fix up limits in dpm_init()

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