basesink: clean up the need_preroll variable
basesink: clean up the need_preroll variable

Based on patch from Song Bing <b06498@freescale.com>

Don't just set the need_preroll flag to TRUE in all cases. When we
are already prerolled it needs to be set to FALSE and when we go to
READY we should not touch it. We should only set it to TRUE in other
cases, like what the code above does.

See https://bugzilla.gnome.org/show_bug.cgi?id=736655

krichter722/gstreamer,mparis/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,mparis/gstreamer,jpxiong/gstreamer,mparis/gstreamer,jpxiong/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,StreamUtils/gstreamer,mparis/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,centricular/gstreamer,mparis/gstreamer,jpxiong/gstreamer