FBC is not applicable for the underlay pipe,
hence disallow enabling and disabling of the same.

This also fixes the BUG hit of calling sleep in
atomic context.
Signed-off-by: Shirish S <shirish.s@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>