drm/amd/display: Clear encoder assignment for copied streams

[Why]
When copying a stream, the encoder assigned to it is copied too.
Encoder assignment should only happen when executing the encoder
assignment function link_encs_assign().

[How]
Clear the link encoder pointer for copied stream.

Reviewed-by: Meenakshikumar Somasundaram <Meenakshikumar.Somasundaram@amd.com>
Reviewed-by: Jun Lei <Jun.Lei@amd.com>
Acked-by: Agustin Gutierrez Sanchez <agustin.gutierrez@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>