drm/amd/display: destroy panel on link destruct

[Why]
without destroy it is causing a memory leak

[How]
destroy panel on link destruct

Signed-off-by: Anthony Koo <Anthony.Koo@amd.com>
Reviewed-by: Wyatt Wood <Wyatt.Wood@amd.com>
Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>