drm/amd/display: add assert in enable FBC

Adding assert to prevent possible null deref warning.
Only can happen under abnormal circumstances.

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