drm/amd/display: Disable FEC when powering down encoders

drm/amd/display: Disable FEC when powering down encoders

[why & how]
VBIOS DMCUB FW can enable FEC for capable eDPs, but S/W DC state is
only updated for link0 when transitioning into OS with driver loaded.
This causes issues when the eDP is immediately hidden and DIG0 is
assigned to another link that does not support FEC. Driver will
attempt to disable FEC but FEC enablement occurs based on the link
state, which does not have fec_state updated since it is a different
link. Thus, FEC disablement on DIG0 will get skipped and cause no
light up.

Reviewed-by: Karen Chen <karen.chen@amd.com>
Signed-off-by: Ovidiu Bunea <ovidiu.bunea@amd.com>
Signed-off-by: Matthew Stewart <matthew.stewart2@amd.com>
Tested-by: Dan Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>