drm/radeon/kms: use tracked values for sclk and mclk

Rather than calling get_memory_clock and get_engine_clock,
used the tracked values from the pm code.  Calling the tables
adds additional latency in the modesetting and pm paths.

Signed-off-by: Alex Deucher <alexdeucher@gmail.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>