drm/amd/display: Fix dynamic link encoder access.

[Why]
Assuming DIG link encoders are statically mapped to links can cause
system instability due to null pointer accesses.

[How]
- Add checks for non-null link encoder pointers before trying to access
them.
- When a hardware platform uses dynamic DIG assignment (i.e. resource
function 'link_encs_assign' defined) and a link supports flexible
mapping to DIGs, use the link_enc_cfg API to access the DIG assigned to
a link or stream.

Reviewed-by: Meenakshikumar Somasundaram <meenakshikumar.somasundaram@amd.com>
Acked-by: Mikita Lipski <mikita.lipski@amd.com>
Signed-off-by: Jimmy Kizito <Jimmy.Kizito@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>