drm/amd/display: guard for virtual calling destroy_link_encoders

[Why]:
On power down, virtual dal may try to delete link_encoders by
referencing uninitialized res_pool.

[How]:
Added guard against empty res_pool.

Acked-by: Rodrigo Siqueira <Rodrigo.Siqueira@amd.com>
Signed-off-by: Martin Leung <martin.leung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>