drm/amd/display: Remove incorrect ASSERT check for link_enc

[Why]
It's possible that we don't have a link encoder assignment if the
context is NULL but we're calling dc_add_stream_to_ctx from DM directly.

Link encoder assignment will happen later after global validation
runs with fast_validate = false.

[How]
Remove the ASSERTION. We already guard against NULL link_enc.

Reviewed-by: Michael Strauss <Michael.Strauss@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Nicholas Kazlauskas <nicholas.kazlauskas@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>