lavfi/procamp_vaapi: fix the green video issue if without arguments.

Fix the green output issue when use procamp_vaapi without any
arguments, now if use procamp_vaapi without any arguments, will use
the default value to setting procamp_vaapi.

Signed-off-by: Jun Zhao <jun.zhao@intel.com>
Signed-off-by: Mark Thompson <sw@jkqxz.net>
