drm/amd/powerplay: Add floor DCEF for DS on boot.

Use the vbios to look up the default frequencies
for socclk and dcefclk.

Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>