vaapidecode: return pad's template caps if no display

vaapidecode: return pad's template caps if no display

A caps query can occur before the element has a display. In that case, the
element can return its pad's template. But when the element already has a
display, and the caps probe fails, the element shall return an empty caps, so
the auto-plug could try with another decoder.

If the element has a display and the caps probe works, then the computed caps
should be returned.

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