drm/radeon: don't use get_engine_clock() on APUs

It doesn't work reliably.  Just report back the currently
selected engine clock.

Partially fixes:
https://bugs.freedesktop.org/show_bug.cgi?id=62493



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