drm/amd/display: Add missing mclk update

When using FPO, there is some misconfiguration that happens for the lack
of configuration of the MCLK switch in some circumstances. This commit
adds the required field update when using the MCLK switch.

Reviewed-by: Aurabindo Pillai <aurabindo.pillai@amd.com>
Signed-off-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>