drm/amd/powerplay: fix arcturus real-time clock frequency retrieval

Make sure we can still get the accurate gfxclk/uclk/socclk frequency
even on dpm disabled.

Signed-off-by: Evan Quan <evan.quan@amd.com>
Reviewed-by: Kenneth Feng <kenneth.feng@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>