vaapipostproc: don't set caps change at first set

vaapipostproc: don't set caps change at first set

When the source caps change, the filter is destroyed and recreated.
Nonetheless, this happens every time the vaapipostproc starts, since the caps
change detection algorithm does not take in consideration when the caps are
set by first time.

This patch intents to be an optimization, to avoid a useless filter
destroy-creation cycle when the sources caps are set for first time.

The new helper function video_info_update() is a refactorization to avoid
duplicated code.

Signed-off-by: Víctor Manuel Jáquez Leal <victorx.jaquez@intel.com>
https://bugzilla.gnome.org/show_bug.cgi?id=758007