drm/amd/display: disable FBC on underlay pipe

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>