plugins: fix display type comparison in gst_vaapi_create_display().

plugins: fix display type comparison in gst_vaapi_create_display().

After the code got moved to create the gst_vaapi_create_display() helper,
this comparison was not updated to dereference the newly-created
pointer, so the code was comparing the pointer itself to the type, and
therefore failing to retrieve the VA display.

This fixes the following error (and gets gst-vaapi decoding again):

    ERROR vaapidecode gstvaapidecode.c:807:gst_vaapidecode_ensure_allowed_caps: failed to retrieve VA display

https://bugzilla.gnome.org/show_bug.cgi?id=704410

Signed-off-by: Emilio López <emilio@elopez.com.ar>