plugins: check if display is set in sync

plugins: check if display is set in sync

Since the context messages are sync'ed, the display assignation happens in the
same thread, hence we can know if the display was found or not as soon we call
for it.

In order to take advantage of it, gst_vaapi_video_context_prepare() receives,
as a new parameter, the address of the plugin's display, and reports back if
the display was found and set.

Signed-off-by: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>
https://bugzilla.gnome.org/show_bug.cgi?id=757598