drm/amd/display: Fix FBC compilation

- Fixing compilation with ENABLE_FBC flag
  which was broken after flattening core_dc to dc

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