drm/amd/display: Enable SYMCLK gating in DCCG

[WHY & HOW]
Enable root clock optimization for SYMCLK and only
disable it when it's actively used.

Reviewed-by: Charlene Liu <charlene.liu@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Daniel Miess <daniel.miess@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>