panvk: expose VK_KHR_display

It seems the common WSI code does all that's really needed here for us
already. Enabling this lets me run vkmark on PanVK.

This is a bit silly, because what actually happens here is that we end
up passing -1 as the display_fd to wsi_device_init(). This in turn leads
us to returning zero usable displays, which renders the extension
somewhat useless. But it is better than not supporting the extension, and
not supporting applications who have a hard depdendency on it fail, like
is the case with vkmark.

Tested-by: Alexandre ARNOUD <aarnoud@me.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Part-of: <!33916>