panvk: Support modifiers for Wayland WSI

If we don't send modifiers, Wayland compositors end up with DRM_FORMAT_MOD_INVALID which may prevent them from e.g. assigning an overlay plane to an application.

Tested on RK3566 Mali G52 and by setting this, we start getting applications assigned to planes for Vulkan applications in weston (as already also works for OpenGL with panfrost). I understand that it might be unset just because nobody enabled it and tested it yet, or was there another reason for it?