zink: add get_video_param() and is_video_format_supported() stubs

Simple stubs to prevent crashes in apps, based on feedback on !31528 (closed)

vainfo, spectacle, and chromium based apps crashed/segfaulted without these. With this commit, they all run fine

[sidpr@infiltrator mesa-git]$ vainfo
Trying display: wayland
vainfo: VA-API version: 1.22 (libva 2.22.0)
vainfo: Driver version: Mesa Gallium driver 24.3.0-devel for zink Vulkan 1.3(NVIDIA GeForce RTX 3070 (NVK GA104) (MESA_NVK))
vainfo: Supported profile and entrypoints
      VAProfileNone                   : VAEntrypointVideoProc