drm/amd/powerplay: return current DCEFCLK on sysfs read (v2)

This fixes incorrect output when reading pp_dpm_dcefclk sysfs.

Output before this patch:
cat /sys/devices/pci0000:<snip>/pp_dpm_dcefclk
0: 506Mhz
1: 42949671Mhz *
2: 1266Mhz

v2: squash in arcturus fix

Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>