drm/amd/display: Add null pointer check for get_first_active_display()

The function mod_hdcp_hdcp1_enable_encryption() 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 in
mod_hdcp_hdcp2_enable_encryption().

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

Fixes: 2deade5 ("drm/amd/display: Remove hdcp display state with mst fix")
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Reviewed-by: Alex Hung <alex.hung@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org # v5.8

Author: ISCAS-Vulab

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

@@ -368,6 +368,9 @@ enum mod_hdcp_status mod_hdcp_hdcp1_enable_encryption(struct mod_hdcp *hdcp)
 	struct mod_hdcp_display *display = get_first_active_display(hdcp);
 	enum mod_hdcp_status status = MOD_HDCP_STATUS_SUCCESS;
 
+	if (!display)
+		return MOD_HDCP_STATUS_DISPLAY_NOT_FOUND;
+
 	mutex_lock(&psp->hdcp_context.mutex);
 	hdcp_cmd = (struct ta_hdcp_shared_memory *)psp->hdcp_context.context.mem_context.shared_buf;
 	memset(hdcp_cmd, 0, sizeof(struct ta_hdcp_shared_memory));