encoder: hevc:  fix bug in multi slice encoding.
encoder: hevc:  fix bug in multi slice encoding.

This is a work-around for satisfying the VA-Intel driver.
The driver only support slices begin from CTU row start address.

Multi-Slice encoding also requires a fix in va-intel-driver:
http://lists.freedesktop.org/archives/libva/2015-May/003351.html

https://bugzilla.gnome.org/show_bug.cgi?id=749854

Signed-off-by: Sreerenj Balachandran <8526e100bdc7a12d85af109c9f42b86756e50e67@intel.com>

gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,GStreamer/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,ceyusa/gstreamer-vaapi