vaapidecodebin: ensure VPP before going to READY

vaapidecodebin: ensure VPP before going to READY

There are sometimes that the VA-API display context is not shared among the
pipeline, but it is important to know it before going to READY state (when the
pipeline is already linked).

One instance of this case is this:

gst-launch-1.0 filesrc location=media ! decodebin ! vaapipostproc ! vaapisink

This patch adds a new function in gstvaapipluginutil called
gst_vaapi_create_test_display(). Its purpose is to create a disposable VA-API
display, which only will be used for verify if the VAEntrypointVideoProc is
available by the hardware. Afterwards, it should be unrefed.

If the vaapidecodebin is going to READY state, and the element still doesn't
know if VPP is available, the last resort is to create a new instance of the
VA-API display and test for it.

https://bugzilla.gnome.org/show_bug.cgi?id=749554