drm/amd/display: FEC configuration for dpia links in MST mode

[Why]
To fix the check condition for fec enable for dpia links in MST mode.

[How]
dc_link_should_enable_fec() to be used to check whether fec should be
enabled in MST mode.

Cc: Wayne Lin <wayne.lin@amd.com>
Reviewed-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>