drm/amd/display: Update SW state correctly for FCLK

FCLK not supported for DCN321, but still need to update the software
state accordingly to prevent unneeded full updates in driver

Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>