drm/amd/display: Fix DPIA link encoder assignment issue

[Why]
The DPIA link encoder (DIG) was not released when a Display XDR 6K
monitor was disconnected. That's because the DPIA link encoder logic
checked the BW when it removed the link encoder for it. And the timing
BW was less than DP link BW.  So, it failed to release the link encoder.
And that caused the DIG was occupied and can't be used.  The reason it
checked the BW is it wants to identify whether this link was using DIG
or HPO. It skips if it's not DIG.

[How]
Not to check the BW when it removes the link encoder because it's
already in the assignment table, it must be the DIG.

Tested-by: Daniel Wheeler <Daniel.Wheeler@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Cruise Hung <Cruise.Hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>