drm/amd/display: Init link enc resources in dc_state only if res_pool presents

[Why & How]
res_pool is not initialized in all situations such as virtual
environments, and therefore link encoder resources should not be
initialized if res_pool is NULL.

Cc: Mario Limonciello <mario.limonciello@amd.com>
Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Martin Leung <martin.leung@amd.com>
Acked-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Dillon Varone <dillon.varone@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>