drm/amd/display: Cleanup dce_get_dp_ref_freq_khz

[WHY&HOW]
Cleanup unnecessary code pathes as compile guards were added and removed
overtime.

Reviewed-by: Alvin Lee <alvin.lee2@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>