drm/amd/display: Hardcode vco_freq for dcn316

There is no need to calculate the VCO frequency. In our internal branch
we've hard-coded this for a while, so it's well-tested. This also allows
us to remove the now unused code for calculating the VCO frequency.

Reviewed-by: Harry Wentland <harry.wentland@amd.com>
Acked-by: Alan Liu <haoping.liu@amd.com>
Signed-off-by: Alan Liu <haoping.liu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>