drm/amd/display: Add null pointer check in mod_hdcp_hdcp1_create_session()

The function mod_hdcp_hdcp1_create_session() calls the function
get_first_active_display(), but does not check its return value.
The return value is a null pointer if the display list is empty.
This will lead to a null pointer dereference.

Add a null pointer check for get_first_active_display() and return
MOD_HDCP_STATUS_DISPLAY_NOT_FOUND if the function return null.

This is similar to the commit c3e9826a2202
("drm/amd/display: Add null pointer check for get_first_active_display()").

Fixes: 2deade5ede56 ("drm/amd/display: Remove hdcp display state with mst fix")
Signed-off-by: Chenyuan Yang <[e-mail]>
Reviewed-by: Alex Hung <[e-mail]>
Tested-by: Dan Wheeler <[e-mail]>
Signed-off-by: Alex Deucher <[e-mail]>
(cherry picked from commit 5e43eb3cd731649c4f8b9134f857be62a416c893)

drivers/gpu/drm/amd/display/modules/hdcp/hdcp_psp.c

···
 		return MOD_HDCP_STATUS_FAILURE;
 	}
 
+	if (!display)
+		return MOD_HDCP_STATUS_DISPLAY_NOT_FOUND;
+
 	hdcp_cmd = (struct ta_hdcp_shared_memory *)psp->hdcp_context.context.mem_context.shared_buf;
 
 	mutex_lock(&psp->hdcp_context.mutex);