drm/amd/display: add null check

[WHY]
Prevents null pointer dereferences to enhance function robustness

[HOW]
Adds early null check and return false if invalid.

Reviewed-by: Cruise Hung <cruise.hung@amd.com>
Signed-off-by: Peichen Huang <PeiChen.Huang@amd.com>
Signed-off-by: Ray Wu <ray.wu@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>

Author: Peichen Huang

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

@@ -6398,11 +6398,13 @@ unsigned int dc_get_det_buffer_size_from_state(const struct dc_state *context)
  */
 bool dc_get_host_router_index(const struct dc_link *link, unsigned int *host_router_index)
 {
-	struct dc *dc = link->ctx->dc;
+	struct dc *dc;
 
-	if (link->ep_type != DISPLAY_ENDPOINT_USB4_DPIA)
+	if (!link || !host_router_index || link->ep_type != DISPLAY_ENDPOINT_USB4_DPIA)
 		return false;
 
+	dc = link->ctx->dc;
+
 	if (link->link_index < dc->lowest_dpia_link_index)
 		return false;