drm/radeon/dpm: fix fallback for empty UVD clocks

Some older 6xx-7xx boards didn't always fill in the
UVD clocks properly in the UVD power states.  This
leads to the driver trying to set a 0 clock which
results in slow or broken UVD playback.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=69120



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