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

In the Linux kernel, the following vulnerability has been resolved:

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.