drm/amd/display: only include FEC overhead if both asic and display support FEC

[why]
Some asics don't support FEC but FEC overhead is added into link
bandwidth calculation by mistake. This causes certain timing cannot be
validated.

[how]
Only include FEC overhead if both asic and display support FEC.

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>