multimedia/mpv: backport --gpu-context=displayvk

Similar to vkcube-display and --gpu-context=drm (OpenGL). For example,

$ mpv --vo=gpu --gpu-api=vulkan --gpu-context=displayvk --vulkan-display-plane=2 /path/to/video

I tested on the following configuration (consult --vulkan-display-* in the manpage)

$ mpv --vulkan-display-plane=help
Vulkan Devices:
  'Intel(R) HD Graphics 530 (SKL GT2)' (GPU ID 8086:1912)
    Display 0: 'monitor' (3840x2160)
    Modes:
      Mode 00: 3840x2160 (60.000 Hz)
      Mode 01: 3840x2160 (30.000 Hz)
      Mode 02: 1920x1080 (60.000 Hz)
      Mode 03: 1920x1080 (59.940 Hz)
      Mode 04: 1600x900 (60.000 Hz)
      Mode 05: 1280x1024 (60.020 Hz)
      Mode 06: 1280x800 (59.810 Hz)
      Mode 07: 1152x864 (59.972 Hz)
      Mode 08: 1280x720 (60.000 Hz)
      Mode 09: 1280x720 (59.940 Hz)
      Mode 10: 1024x768 (60.004 Hz)
      Mode 11: 800x600 (60.317 Hz)
      Mode 12: 720x480 (60.000 Hz)
      Mode 13: 720x480 (59.940 Hz)
      Mode 14: 640x480 (60.000 Hz)
      Mode 15: 640x480 (59.940 Hz)
    Planes:
      Plane: 2