drm/amd/display: Add NULL check

[Why]
Virtualization enters blue screen of death (BSoD)
due to NULL res_pool object when accessing DSC
encoder capability.

[How]
Add NULL check to avoid blue screen of death.

Reviewed-by: Aric Cyr <Aric.Cyr@amd.com>
Acked-by: Alan Liu <HaoPing.Liu@amd.com>
Signed-off-by: Chris Park <Chris.Park@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>