drm/radeon/r6xx: add a stubbed out set_uvd_clocks callback

Certain r6xx boards use the same power state for both UVD
and other things.  Since we don't support UVD on r6xx boards
at the moment, there was no callback installed for setting
the UVD clocks, however, on systems that use the same power
state, this leads to a NULL pointer dereference.  Fill
in a stubbed out implementation for now to avoid the crash.

Fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=66963

Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: "3.11" <stable@vger.kernel.org>