drm/amd/display: separate FEC capability from fec debug flag

[why]
FEC capability query should not be affected by debugging decision on
whether to disable FEC. We should not determine if display supports FEC
by checking debug option.

Signed-off-by: Wenjing Liu <Wenjing.Liu@amd.com>
Reviewed-by: Ashley Thomas <Ashley.Thomas2@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>