test: fix a ninja test failure for vaapioverlay.
That test case will fail when build with
meson -Dwith_x11=yes -Dwith_wayland=no -Dwith_drm=no -Dwith_egl=yes -Dwith_glx=no
Get a failure:
general:test_overlay_position:0: Unexpected critical/warning: value "((GstVaapiDisplayType) 4)" of type 'GstVaapiDisplayType' is invalid or out of range for property 'dis play' of type 'GstVaapiDisplayType'
The display type changes when build with different paramters, we just need to set the display type to a default value.