plugins: fix context query handling

plugins: fix context query handling

The current context query handling design is flawed: the function
gst_vaapi_reply_to_query() returns FALSE either if the query is not a
GST_CONTEXT_QUERY of if the query could not be handled correctly. But the
pad query function should handle differently each case.

This patch changes the gst_vaapi_reply_to_query() for
gst_vaapi_handle_context_query() and changes it usage in all the vaapi plugins
to match the correct context query handling.

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