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>

drivers/gpu/drm/amd/display/dc/core/dc_resource.c

@@ -2335,6 +2335,9 @@ void dc_resource_state_construct(
 
 bool dc_resource_is_dsc_encoding_supported(const struct dc *dc)
 {
+	if (dc->res_pool == NULL)
+		return false;
+
 	return dc->res_pool->res_cap->num_dsc > 0;
 }