drm/amd/display: Add FBC on/off to front-end programming

- Fixing text mode for cases when VT-switch doesn't result
in timing change

Signed-off-by: Roman Li <Roman.Li@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>