drm/amd/display: Fix FBC disable for stereo

Removed FBC disable from dc_enable_stereo().
It should be implemented in stereo_setup callback, which is not
implemented yet. Otherwise it always disable FBC regardless
if stereo supported or not.

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