mpv: va-api requires egl-drm, wayland and/or x11

mpv: va-api requires egl-drm, wayland and/or x11

The VA-API acceleration requires one or more of the egl-drm, wayland
and/or x11 backends.
Since the egl-drm backend means LIBGL (X11) + EGL + DRM we'll handle it
later once the mesa3d libgl hidden knob is applied.
So for now we'll support wayland and x11 until that happens. Fixes:
http://autobuild.buildroot.net/results/4b9/4b98d0c2c0617715e77fb47b3d97037c6b7562ec/

[Peter: pass --disable-vaapi if libva isn't enabled,
 invert wayland/X11 test as suggested by Thomas]
Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>