drm/amd/display: handle null link encoder

[Why]
The link encoder mapping could return a null one and causes system crash.

[How]
Let the mapping can get an available link encoder without endpoint
identification check.

Reviewed-by: Wenjing Liu <Wenjing.Liu@amd.com>
Acked-by: Jasdeep Dhillon <jdhillon@amd.com>
Signed-off-by: Martin Tsai <martin.tsai@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>